V4L/DVB (12326): zr364xx: error message when buffer is too small and code cleanup

. added code to print an error message when buffer is too small to hold frame
data, that is better than just a hard crash. Tested using MAX_FRAME_SIZE =
50000, lots of error messages appeared in /var/log/messages but no crash.

. removed line "f->type = V4L2_BUF_TYPE_VIDEO_CAPTURE;" in
zr364xx_vidioc_try_fmt_vid_cap, it should not be there.

. changes to improve performance (or at least not hurt it): removed some
unneeded debug messages; added macro FULL_DEBUG to enable debug messages in
performance critical places, this macro is disabled by default; removed "if
(frm->lpvbits == NULL)..." in zr364xx_read_video_callback because after
analisying the source code I concluded it will always results to false, so it
is not needed.

. some small code reorganization.

Signed-off-by: Lamarque V. Souza <lamarque@gmail.com>
Signed-off-by: Antoine Jacquet <royale@zerezo.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>