V4L/DVB (4275): The FE_SET_FRONTEND_TUNE_MODE ioctl always returns EOPNOTSUPP

When someone added the front-end ioctl FE_SET_FRONTEND_TUNE_MODE, they
forgot to set the return value to 0.  It always returns EOPNOTSUPP,
causing problems for programmers who actually check for error conditions.

Signed-off-by: Trent Piepho <xyzzy@speakeasy.org>
Signed-off-by: Andrew de Quincey <adq_dvb@lidskialf.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>