usb: core: Fix use after free for hub usb device

There is a possibility of root hub usb device being
freed as part of controller driver unbind and hub
thread trying to access root hub usb device. Hence
increment the usb device reference count before
accessing it by hub thread.

CRs-Fixed: 671581
Change-Id: Icf1297452e55a4ca0acd67075320f7a32929a1b3
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>