[nrf528xx] return the TX power which was actually set in the register (#4451)

This commit ensures that TX Power returned via
otPlatRadioGetTransmitPower is one that is actually set in the
register. E.g. even if TX power is set to 100 via
otPlatRadioSetTransmitPower, the maximum power is 8 for nRF52840.

examples/platforms/nrf528xx/src/radio.c

@@ -571,7 +571,7 @@ otError otPlatRadioGetTransmitPower(otInstance *aInstance, int8_t *aPower)
     }
     else
     {
-        *aPower = sDefaultTxPower;
+        *aPower = nrf_802154_tx_power_get();
     }
 
     return error;