Fix sparse warning:
	drivers/media/usb/em28xx/em28xx-cards.c:69:1: sparse: symbol 'em28xx_devused' 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>