ALSA: pcm: Remove down_write() hack for snd_pcm_link_rwsem

Remove the hackish down_write_nonfifo() that was introduced as a
workaround of rwsem deadlock.

It used to be a problem for non-atomic PCM streams that take the rwsem
for the locking and hit the high lock contention.  Since the current
PCM locking refactoring, we'll no longer hit it as the hot code-paths
don't take global locks.

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