V4L/DVB (9911): em28xx: vidioc_try_fmt_vid_cap() doesn't need any lock

vidioc_try_fmt_vid_cap() just checks if a given resolution is supported.
It doesn't touch on struct em28xx device descriptor. so, there's no need
to lock.

While there, use unlikely() for those values that aren't likely to
occur.

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