[ALSA] usbaudio - Fix Oops with unconventional sample rates

The patch fixes the memory corruption by the support of unconventional
sample rates.  Also, it avoids the too restrictive constraints if
any of usb descriptions contain continuous rates.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>