[media] bttv: Width must be a multiple of 16 when capturing planar formats

On my bttv card "Hauppauge WinTV [card=10]" capturing in YV12 fmt at max
size results in a solid green rectangle being captured (all colors 0 in
YUV).

This turns out to be caused by max-width (924) not being a multiple of 16.

We've likely never hit this problem before since normally xawtv / tvtime,
etc. will prefer packed pixel formats. But when using a video card which
is using xf86-video-modesetting + glamor, only planar XVideo fmts are
available, and xawtv will chose a matching capture format to avoid needing
to do conversion, triggering the solid green window problem.

Cc: stable@vger.kernel.org
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>