V4L/DVB (5131): M920x: more cleanups

Some cleanups and suggestions from Patrick Boettcher.
Dropped the mutex in m9206_rc_query using #if 0, because
M9206_CORE, M9206_I2C, M9206_FILTER and M9206_FW can be accessed
concurrently.
Thanks to both Aapo Tahkola and Patrick Boettcher.

Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>