staging: bcm2835-audio: Drop superfluous mutex lock during prepare

The chip->audio_mutex is used basically for protecting the opened
stream assignment, and the prepare callback is irrelevant with it.

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>