V4L/DVB: ov7670: silence some compiler warnings

Get rid of some "unused variable" warnings.  These were the result of
sloppiness and should not have happened; I'll go stand in the corner now.

Reported-by: Mauro Carvalho Chehab <mchehab@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>