[media] em28xx: make 'em28xx_ctrl_ops' static

sparse warnings: (new ones prefixed by >>)

>> drivers/media/usb/em28xx/em28xx-video.c:1151:28: sparse: symbol 'em28xx_ctrl_ops' was not declared. Should it be static?

Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>