[media] adv7604: Don't put info string arrays on the stack

We do not want to modify the info string arrays ever, so no need to
waste stack space for them. While we are at it also make them const.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>