extcon: usb-gpio: fix a non-kernel-doc comment

Do not use "/**" to begin a non-kernel-doc comment.
Fixes this build warning:

drivers/extcon/extcon-usb-gpio.c:23:
	warning: expecting prototype for drivers/extcon/extcon-usb-gpio.c().

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>

drivers/extcon/extcon-usb-gpio.c

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: GPL-2.0-only
-/**
+/*
  * drivers/extcon/extcon-usb-gpio.c - USB GPIO extcon driver
  *
  * Copyright (C) 2015 Texas Instruments Incorporated - https://www.ti.com