ENOTTY is now returned for unimplemented ioctl by dvb-frontend.
Old EOPNOTSUPP & ENOSYS could be still returned by some drivers
as well as other "non standard" error codes.

EAGAIN is returned in case of device is in state where it cannot
perform requested operation. This is for example sleep and statistics
are queried. Quick check for few demodulator drivers reveals there is
a lot of different error codes used in such case currently, few to
mention still: EOPNOTSUPP, ENOSYS, EAGAIN ... Lets try harmonize.
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>