WUSB: fix oops when completing URBs for disconnected devices

Fix an oops in wusbhc_giveback_urb() if the wusb device had disconnected
while an urb was in progress. Also release the ref count obtained here.

Signed-off-by: David Vrabel <david.vrabel@csr.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>