[media] V4L: soc-camera: switch to using the existing .enum_framesizes()

The recently introduced .enum_mbus_fsizes() v4l2-subdev video operation is
a duplicate of the .enum_framesizes() operation, introduced earlier. Switch
soc-camera over to using the original one.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>