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

Change-Id: I0e67a74a84416e6325725fc4b7d907d3c89a70e1
Signed-off-by: Ruslan Bilovol <ruslan.bilovol@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Git-commit: d355339eecd986648420e05f8c958fbc78dbb382
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git


[ajaya@codeaurora.org: resolve trivial merge conflicts]
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>