V4L/DVB (5249): Fix compiler warning in vivi.c

The result of copy_to_user was not used, so the compiler complained
now a warning will be issued if copy_to_user fails.

Signed-off-by: Marcel Siegert <mws@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>