Return 0 instead of -EINVAL in get_frontend if no demod mode is active.
This fixes ie. dvb-fe-tool getting confused and assuming a DVBv3 FE on idle
frontends when the FE has been put to sleep using sleep().

Also, in read_status(), don't immediately return when no demod is active,
so the remaining code has a chance to clear the signal statistics.
Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>