V4L/DVB (12169): em28xx-video: fix VIDIOC_G_FMT and VIDIOC_ENUMFMT with webcams

Webcams have different constraints than other v4l devices. This patch
makes the format ioctls to behave better. It also fixes a bug at open()
handler, that were always reseting resolution to the maximum available
one.

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