usb: gadget: f_uac2: uevent changes for uac2

adding uvent from kernel audio gadget driver on
usb disconnect/connect, which userspace needs to
listen and then read the sysfs entry exposed by
this change, to know if usb is connected or not.

Change-Id: I5782691c0ae7b78d953cde8ac24980bc3e2110ce
Signed-off-by: Vijayavardhan Vennapusa <quic_vvreddy@quicinc.com>
Signed-off-by: AKASH KUMAR <quic_akakum@quicinc.com>