em28xx: don't reduce scale to half size for em2800

Since em2800 can't support 720x480 / 720x576, the driver used to reduce
the scale to half the size on those chips. As the proper fix were
applied, reducing the maximum horizontal resolution to 640, this hack
can be removed.

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