[media] fc0012: remove unused callback and correct one comment

There is no need to keep dummy sleep() callback implementation as
DVB-core checks existence of it before calls callback. Due to that
we can remove it.
FC0012 is based of direct-conversion receiver architecture
(aka Zero-IF) where is no IF used. Due to that IF is always 0 Hz.
Fix comment to point that.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Acked-by: Hans-Frieder Vogt <hfvogt@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>