em28xx: em2800 chips support max width of 640

Due to hardware limitation, em2800 chips can't work at resolutions
higher than 640x576, since the URB packet size is not enough.

The effect is that the image looses packages and shows a distortion
along the vertical axes.

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