[ALSA] Change AT91 PDC register defines for 2.6.20 kernel

Use the new PDC register name defines that were updated
in Linux 2.6.20.

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>