tx_power_offset wrong binding

## Bug

The device creation macro for the `lr11xx` and `sx126x` boards uses the wrong naming for the `tx_offest` property. It is defined as `tx-power-offset` in the `semtech,lr11xx-common.yaml` and `semtech,sx126x-new-common.yaml` files; however, the binding uses `tx_offset` instead of `tx_power_offset`.

## Fix

Replace `tx_offset` with `tx_power_offset` in `lr11xx_board.c` and `sx126x_board.c`.