[ALSA] protect Dreamcast PCM driver (AICA) from G2 bus effects

The G2 bus on the SEGA Dreamcast connects both the maple peripheral
bus and the AICA sound memory. DMA requests on one can cause the other
to timeout on memory operations.
This patch prevents maple interrupts from causing hiccoughs in the
AICA sound (maple bus code will land in 2.6.24).
There are other cleanups for this (AICA) code - but this is in effect
a regression fix rather than a cleanup.

Signed-off-by: Adrian McMenamin <adrian@mcmen.demon.co.uk>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>