usb: gadget: function: uvc: fix sparse warnings

this patch fixes following sparse warnings:

uvc_video.c:283:5: warning: symbol 'uvcg_video_pump' was not declared. Should it be static?
uvc_video.c:342:5: warning: symbol 'uvcg_video_enable' was not declared. Should it be static?
uvc_video.c:381:5: warning: symbol 'uvcg_video_init' was not declared. Should it be static?

Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>