unittests: Test_LoRaMac.post_process_mcps_req: Fix segfault

We had commented out a line where we reset LoRaPHY_stub::uint16_value to
0. This was causing an invalid array access in
LoRaMac::handle_data_frame, when trying to extract the
_mps_indication.channel from list of channel_params_t returned by
lora_phy->get_phy_channels.

Author: rwalton-arm

connectivity/lorawan/tests/UNITTESTS/features/lorawan/loramac/Test_LoRaMac.cpp

@@ -432,7 +432,7 @@ TEST_F(Test_LoRaMac, post_process_mcps_req)
     LoRaMacCrypto_stub::int_table_idx_value = 0;
     LoRaMacCrypto_stub::int_table[0] = 4;
     LoRaMacCrypto_stub::int_table[1] = 4;
-//    LoRaPHY_stub::uint16_value = 0;
+    LoRaPHY_stub::uint16_value = 0;
     object->on_radio_rx_done(payload, 16, 0, 0);
     object->post_process_mcps_req();