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>

sound/usb/Kconfig

@@ -160,5 +160,7 @@ config SND_BCD2000
 	  To compile this driver as a module, choose M here: the module
 	  will be called snd-bcd2000.
 
+source "sound/usb/line6/Kconfig"
+
 endif	# SND_USB