usb: gadget: function: make current f_uac1 implementation legacy

Before introducing new f_uac1 function (with virtual
ALSA card) make current implementation legacy.

This includes renaming of existing files, some
variables, config options and documentation

Signed-off-by: Ruslan Bilovol <ruslan.bilovol@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>