usb: gadget: diag: Notify cable status while opening channel

Client will miss cable connect notification if USB cable is
already connected before channel is opened. Notify client
through notify callback if usb cable is already connected
when channel is opened.

Change-Id: I5f94cd8c572522edb575d498fa2108c1a5b938c8
Signed-off-by: Vamsi Krishna Samavedam <vskrishn@codeaurora.org>