ALSA: usb-audio: fix Line6 Helix audio format rates

Line6 Helix and HX stomp devices don't support retrieving
the number of clock sample rate.

Add a quirk to set it to 48Khz by default.

[ fixed wrong variable initialization changes by tiwai ]

Signed-off-by: Nicola Lunghi <nick83ola@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>