staging: bcm2835-audio: Clean up mutex locks

snd-bcm2835 driver takes the lock with mutex_lock_interruptible() in
all places, which don't make sense.  Replace them with the simple
mutex_lock().

Also taking a mutex lock right after creating it for each PCM object
is nonsense, too.  It cannot be racy at that point.  We can get rid of
it.

Last but not least, initializing chip->audio_mutex at each place is
error-prone.  Initialize properly at creating the chip object in
snd_bcm2835_create() instead.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>