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>