dwc3: gadget: Take copy of dwc->gadget_driver before releasing lock

gadget_driver can become null if gadget_stop is called while any other
gadget callbacks are in progress. As gadget callbacks needs to release
spinlock before the callback, store the local copy of gadget_driver to
avoid the race with gadget_stop.

Change-Id: I7f0cbf9af3e3b286f2826647f08215f29f699de1
Signed-off-by: Mayank Rana <mrana@codeaurora.org>