usb: wusbcore: fix kernel panic on HWA unplug

This patch adds ref counting to sections of code that operate on struct
wa_xfer objects that were missing it.  Specifically, error handling
cases need to be protected from freeing the xfer while it is still in
use elsewhere.  This fixes a kernel panic that can occur when pulling
the HWA dongle while data is being transferred to a wireless device.

Signed-off-by: Thomas Pugliese <thomas.pugliese@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>