usb: gadget: add "usb_validate_langid" function

The USB LANGID validation code in "check_user_usb_string" function is
moved to "usb_validate_langid" function which can be used by other usb
gadget drivers.

Signed-off-by: Tao Ren <rentao.bupt@gmail.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>