staging: bcm2835-audio: Remove redundant substream mask checks

The avail_substreams bit mask is checked for the possible racy
accesses, but this cannot happen in practice; i.e. the assignment and
the check are superfluous.

Let's rip them off.

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>