diag: Add usb events to a queue

Add usb connect and disconnect events to a queue and
process each event in work function so that all of the
usb connect and disconnect events are processed without
any miss.

Change-Id: I2b5debef28d683f55a727e53e41c811419d2bf3f
Signed-off-by: Sreelakshmi Gownipalli <sgownipa@codeaurora.org>