[media] em28xx-cards: em28xx_devused can be static

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

drivers/media/usb/em28xx/em28xx-cards.c

@@ -66,7 +66,7 @@ MODULE_PARM_DESC(usb_xfer_mode,
 
 
 /* Bitmask marking allocated devices from 0 to EM28XX_MAXBOARDS - 1 */
-DECLARE_BITMAP(em28xx_devused, EM28XX_MAXBOARDS);
+static DECLARE_BITMAP(em28xx_devused, EM28XX_MAXBOARDS);
 
 struct em28xx_hash_table {
 	unsigned long hash;