[media] rc/keymaps: use KEY_CAMERA for snapshots

On a few places, KEY_MHP were used for snapshots. However, KEY_CAMERA
is used for it on all the other keyboards that have a snapshot/Picture
button.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
Signed-off-by: Jarod Wilson <jarod@redhat.com>