usb: gadget: f_uvc: use usb_gstrings_attach

Attach strings to gadget with usb_strings_attach.
It is required for correct instantiation of functions more than once:
instead of modifying the local uvc_en_us_strings a function instance
specific copy is created with usb_gstrings_attach.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Tested-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>