[media] dvb-frontends/drxk: don't log errors on unsupported operation mode

When fe_ops.read_status is called and no channel is tuned (yet), the
subsequent calls to get_lock_status() causes the kernel log to be filled
with

    drxk: Error -22 on get_lock_status

which either means a NULL pointer was passed for the p_lock_status var,
or neither QAM nor OFDM/DVBT operation mode are active. Instead of
filling the kernel log in the latter case, print out a message to the debug
level and return 0 (this isn't used in the calling drxk_get_stats() anyway).

Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>