drivers: ieee802154: ieee802154_uart_pipe: replace magic number

Replaces a magic number by the appropriate definition.

Signed-off-by: Florian Grandel <fgrandel@code-for-humans.de>

drivers/ieee802154/ieee802154_uart_pipe.c

@@ -401,4 +401,4 @@ static const struct ieee802154_radio_api upipe_radio_api = {
 NET_DEVICE_DT_INST_DEFINE(0, upipe_init, NULL, &upipe_context_data, NULL,
 			  CONFIG_KERNEL_INIT_PRIORITY_DEFAULT, &upipe_radio_api,
 			  IEEE802154_L2, NET_L2_GET_CTX_TYPE(IEEE802154_L2),
-			  125);
+			  IEEE802154_MTU);