USB: fix BUG: sleeping function called from invalid context at /home/jeremy/hg/xen/paravirt/linux/drivers/usb/core/urb.c:524, in_atomic():1, irqs_disabled():0

Clearly there's a bug in
drivers/usb/serial/usb-serial.c:usb_serial_put().  It shouldn't call
kref_put() while holding a spinlock.
Signed-off-by: Oliver Neukum <oneukum@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>