[media] pxa_camera: Fix incorrect test in the image size generation

During the transfer from the soc_camera a test in pxa_mbus_image_size()
got removed. Without it any PXA_MBUS_LAYOUT_PACKED format causes either
the return of a wrong value (PXA_MBUS_PACKING_2X8_PADHI doubles
the correct value) or EINVAL (PXA_MBUS_PACKING_NONE and
PXA_MBUS_PACKING_EXTEND16). This was observed in an error from the ffmpeg
(for some of the YUYV subvariants).

This patch re-adds the same test as in soc_camera version.

Signed-off-by: Petr Cvek <petr.cvek@tul.cz>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>