ALSA: usb-audio: More validations of descriptor units

Introduce a new helper to validate each audio descriptor unit before
and check the unit before actually accessing it.  This should harden
against the OOB access cases with malformed descriptors that have been
recently frequently reported by fuzzers.

The existing descriptor checks are still kept although they become
superfluous after this patch.  They'll be cleaned up eventually
later.

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