Bluetooth: Fix potential NULL-pointer access in UART driver

If we get an invalid packet type here the buf pointer will be NULL.
Going to the cleanup section at the end of the function would trigger
a bt_but_put() call which would cause a NULL pointer access. Directly
returning from the function is the right thing to do instead.

Change-Id: I0c18646e0820cf829ef8aa3f77835ba0a14375b5
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>

drivers/bluetooth/uart.c

@@ -161,7 +161,7 @@ void bt_uart_isr(void *unused)
 					break;
 				default:
 					BT_ERR("Unknown H4 type %u\n", type);
-					goto failed;
+					return;
 			}
 
 			if (remaining < 0) {