diag: Defer usb connect/disconnect processing

USB diag driver will be moving connect/disconnect
event notification from process context to interrupt
context. In order to incorporate that, defer usb
connect/disconnect processing in workqueue context.

CRs-Fixed: 475955
Change-Id: I72f9b5330ac81c78a112212d6bedbe27ddd274cb
Signed-off-by: Mohit Aggarwal <maggarwa@codeaurora.org>