media: saa7146: Avoid using BUG_ON as an assertion

In video_begin and video_end, saa7146_format_by_fourcc can return
NULL and is checked via BUG_ON. However, by returning the error
to the callers upstream, we can avoid the crash and handle it via
recovery code.

Signed-off-by: Aditya Pakki <pakki001@umn.edu>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>