ALSA: usx2y: Fix shmem initialization

Currently us428ctls_shmem pages are allocated dynamically upon the
mmap call, but this is quite racy.  Since the shared memory itself is
mandatory for the mmap, let's allocate it at the beginning of the card
initialization.  Also, fix the initialization of the wait queue, too.

Link: https://lore.kernel.org/r/20210517131545.27252-9-tiwai@suse.de


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