usb: gadget: f_uac2: Adds support for uevents

Adds support for uevents to notify the user space
application when the HOST changes the parameters
of the PLAYBACK/CAPTURE stream and also notify the
user when the USB audio function is disabled.

Change-Id: Ic4bd6cee6043913cec3b2ddac950d45c33f6e38f
Signed-off-by: Sriharsha Allenki <sallenki@codeaurora.org>