usb: gadget: fix langid kernel-doc warning in usbstring.c

Fix spelling of the 'langid' function argument in the kernel-doc
notation to quieten a kernel-doc warning.

../drivers/usb/gadget/usbstring.c:77: warning: Function parameter or member 'langid' not described in 'usb_validate_langid'
../drivers/usb/gadget/usbstring.c:77: warning: Excess function parameter 'lang' description in 'usb_validate_langid'

Fixes: 17309a6a ("usb: gadget: add "usb_validate_langid" function")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Tao Ren <rentao.bupt@gmail.com>
Cc: Tao Ren <rentao.bupt@gmail.com>
Cc: Felipe Balbi <balbi@kernel.org>
Signed-off-by: Felipe Balbi <balbi@kernel.org>