media: dvb-frontends/stv0910: release lock on gate_ctrl() failure

Whenever write_reg() fails to open/close the demod's I2C gate, release the
lock to avoid deadlocking situations. If I2c gate open failed, there's no
need to hold a lock, and if close fails, the mutex_unlock() at the end of
the function is never reached, leaving the mutex_lock in locked state,
which in turn will cause potential for deadlocks. Thus, release the lock
on failure.

While we're touching gate_ctrl(), add some explanation about the need for
locking and the shared I2C bus/gate.

Cc: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>