[ALSA] usbaudio.c: remove unneeded casts

Went rummaging through usbaudio.c and found some castings that
aren't needed as far as I can see.  Part of the KernelJanitors
TODO list.

Signed-off-by: John Daiker <daikerjohn@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>