usb: gadget: f_uac2: Support for mic only use case

Add support for dynamic switch between mic only and
mic, speaker use case for the existing f_uac2 driver.
Mic will be enabled always, user can choose to
disable/enable the speaker usecase using the module
parameter enable_capture.
To enable speaker
echo Y > /sys/module/usb_f_uac2/parameters/enable_capture
setprop sys.usb.config diag,adb,uac2
To disable speaker
echo N > /sys/module/usb_f_uac2/parameters/enable_capture
setprop sys.usb.config diag,adb,uac2

Change-Id: Ic58bac474c6854dfb89c0568b887e9fb4fc6d937
Signed-off-by: Sriharsha Allenki <sallenki@codeaurora.org>