[ALSA] Improve the slots option handling

Fix and improve the slots option handling.  The sound core tries to
find the slot with the given module name first and assign if it's
still available.  If all pre-given slots are unavailable, then try
to find another free slot.

Also, when a module name begins with '!', it means the negative match:
the slot will be given for any modules but that one.

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