[ALSA] Fix mask to stop AT91 SSC clock on shutdown

This patch by Frank Mandarino and Hubert Kahlert fixes a bug in the AT91
SSC (i2s) shutdown code that would erroneously disable other AT91
peripheral clocks.

Signed-off-by: Hubert Kahlert <hkahlert@hk-datentechnik.de>
Signed-off-by: Frank Mandarino <fmandarino@endrelia.com>
Signed-off-by: Liam Girdwood <lg@opensource.wolfsonmicro.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>