ALSA: line6: Add high-speed USB support

This has two parts:
* intervals_per_second setup
  (high speed needs 8000, instead of 1000)
* iso_buffers setup (count of iso buffers depends on
  USB speed, 2 is not enough for high speed)

Signed-off-by: Andrej Krutak <dev@andree.sk>
Signed-off-by: Takashi Iwai <tiwai@suse.de>