diag: Wait until usb diag channel opens during cable connect

If USB cable is connected before diag opens its usb channel,
the cable connect notification is missed for diag USB channel.
USB has now provisioned for a connect notification during USB
open. The patch adds a mechanism for diag to wait on connect
notification until the usb open call is completed and fix for
a possible race condition between connect and disconnect events.

Change-Id: I62c4cb80fec23c2b6f1dc8312ee2a4e0c96c03ac
Signed-off-by: Sreelakshmi Gownipalli <sgownipa@codeaurora.org>
Signed-off-by: Manoj Prabhu B <bmanoj@codeaurora.org>