[202012][mellanox]Fix lpmode set when logical port is larger than 64 (#14137)

This PR is to backport #14138 to 202012.

- Why I did it
In sfplpm API, the number of logical ports is hardcoded as 64. When a system contains more port than this, the SDK APIs would fail with a trace as below

Enabling low-power mode for port Ethernet0... Traceback (most recent call last):
File "/usr/share/sonic/platform/plugins/sfplpmset.py", line 167, in
set_lpmode(handle, cmd, sfp_module)
File "/usr/share/sonic/platform/plugins/sfplpmset.py", line 128, in set_lpmode
SX_MGMT_PHY_MOD_PWR_ATTR_PWR_MODE_E, SX_MGMT_PHY_MOD_PWR_MODE_LOW_E)
File "/usr/share/sonic/platform/plugins/sfplpmset.py", line 115, in pwr_attr_set
mgmt_phy_mod_pwr_attr_set(handle, module_id, attr_type, power_mode)
File "/usr/share/sonic/platform/plugins/sfplpmset.py", line 84, in mgmt_phy_mod_pwr_attr_set
assert SX_STATUS_SUCCESS == rc, "sx_mgmt_phy_mod_pwr_attr_set failed"
AssertionError: sx_mgmt_phy_mod_pwr_attr_set failed
Error! Unable to set LPM for 1, rc = 1, err msg: [+] opening sdk
Mar 07 03:25:28 INFO LOG: Initializing SX log with STDOUT as output file.
Mar 07 03:25:28 ERROR SX_API_PORT: sx_mgmt_phy_mod_pwr_attr_get: This API is deprecated and will be removed in the future. Please use sx_mgmt_phy_module_pwr_attr_get in its place.
Mar 07 03:25:28 ERROR SX_API_PORT: sx_mgmt_phy_mod_pwr_attr_set: This API is deprecated and will be removed in the future. Please use sx_mgmt_phy_module_pwr_attr_set in its place.

- How I did it
Remove the hardcoded value of 64. Obtained the number of logical ports from SDK

- How to verify it
Manual testing

Author: dgsudharsan

device/mellanox/x86_64-mlnx_msn2700-r0/plugins/sfplpmset.py

@@ -11,7 +11,6 @@
 
 DEVICE_ID = 1
 SWITCH_ID = 0
-SX_PORT_ATTR_ARR_SIZE = 64
 
 PORT_TYPE_CPU = 4
 PORT_TYPE_NVE = 8
@@ -51,9 +50,13 @@ def set_port_admin_status_by_log_port(handle, log_port, admin_status):
 
 
 def get_log_ports(handle, sfp_module):
-    port_attributes_list = new_sx_port_attributes_t_arr(SX_PORT_ATTR_ARR_SIZE)
     port_cnt_p = new_uint32_t_p()
-    uint32_t_p_assign(port_cnt_p, SX_PORT_ATTR_ARR_SIZE)
+    uint32_t_p_assign(port_cnt_p, 0)
+    rc = sx_api_port_device_get(handle, DEVICE_ID, SWITCH_ID, None,  port_cnt_p)
+
+    assert rc == SX_STATUS_SUCCESS, "sx_api_port_device_get failed, rc = %d" % rc
+    port_cnt = uint32_t_p_value(port_cnt_p)
+    port_attributes_list = new_sx_port_attributes_t_arr(port_cnt)
 
     rc = sx_api_port_device_get(handle, DEVICE_ID, SWITCH_ID, port_attributes_list,  port_cnt_p)
     assert rc == SX_STATUS_SUCCESS, "sx_api_port_device_get failed, rc = %d" % rc