[ALSA] This simplifies and fixes waiting loops of the mce_down()

function after Trent Piepho's patch for AD1848.
It also makes busy_wait() function call not atomic.

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>