media: dvb-frontends/stv0910: WARN_ON() on consecutive mutex_unlock()

Stack dump when gate_ctrl() is called in a way that consecutive unlocks
happen. This is a clear indication that other drivers interfacing with
the stv0910 driver don't do things properly or don't check for failures,
so dump stack so that those drivers can be identified and fixed.

Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Tested-by: Richard Scobie <rascobie@slingshot.co.nz>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>