ALSA: hda - Assign unsol tags dynamically in patch_sigmatel.c

Since we need to handle many unsolicited events assigned to different
widgets, allocate the event dynamically using the existing events
array, and use the tag appropriately instead of combination of fixed
number and widget nid.  (Note that widget nid can be over 4 bits!)

Also, replaced the call of unsol_event handler with a dedicated
function to be more readable.

Signed-off-by: Takashi Iwai <tiwai@suse.de>