V4L/DVB (9230): cx23885: MFE related OOPS fix

Bug: the tree generated an oops when the cx23885 was laoded. This avoids
the oops by ensuring the mutex is correctly initialised before it's used.

Signed-off-by: Steven Toth <stoth@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>