[media] em28xx: increase maxwidth for em2800

The MaxPacketSize for em2800 based devices is too small to capture at full resolution.
Therefore scale down when the maximum frame size is selected.
The previous workaround that simply reduced the X resolution cannot be used
because it crops a part of the input as
the em2800 can only scale down with a factor of 0.5.

reverts commits 1ca31892 and fb3de039

.

[mchehab@redhat.com: Fix CodingStyle]
Signed-off-by: Sascha Sommer <saschasommer@freenet.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>