[media] v4l: s5p-tv: mixer: fix setup of VP scaling

Adjusting of Video Processor's scaling factors was flawed. It bounded scaling
to range 1/16 to 1/1. The correct range should be 1/4 to 4/1. This patch fixes
this bug.

Signed-off-by: Tomasz Stanislawski <t.stanislaws@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>