ALSA: move line6 usb driver into sound/usb

Promote line6 driver from staging to sound/usb/line6 directory, and
maintain through sound subsystem tree.

This commit just moves the code and adapts Makefile / Kconfig.
The further renames and misc cleanups will follow.

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