usb: diag: Set interface protocol to represent diag

PC tools can use this value to identify protocol used
by diag interface. For example, diag interface can use
firehouse or sahara or plain diag protocols. Update the
protocol value to represent plain diag.

Change-Id: Ifba80a21a99575d1fcdb54647319aea0822b8898
Signed-off-by: Vamsi Krishna Samavedam <vskrishn@codeaurora.org>