ALSA: caiaq: Add yet more sanity checks for invalid EPs

A few other places in caiaq driver have the URB handling with the
fixed endpoints without checking the validity, too.  Add the sanity
check with the new helper function at each appropriate place for
avoiding the spurious kernel warnings due to invalid EPs.

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