[media] rc-core: ir_core_dev_number can be static

Fix sparse warning:
	drivers/media/rc/rc-main.c:27:1: sparse: symbol 'ir_core_dev_number' 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>