media: rc-loopback: return number of emitters rather than error

The LIRC_SET_TRANSMITTER_MASK ioctl should return the number of emitters
if an invalid list was set.

Cc: stable@vger.kernel.org
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>