[media] v4l: vs6624: Fix warning due to unused function

vs6624_read() is only called in the conditionally-compiled
vs6624_g_register() function. Make the former conditionally-compiled as
well to silence build warnings.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Scott Jiang <scott.jiang.linux@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>