xhci: fix incorrect type in assignment in handle_device_notification()

This patch converts Event TRB's 3rd field, which has type le32, to CPU
byteorder before using it to retrieve the Slot ID with TRB_TO_SLOT_ID macro.
This bug was found using sparse.

Signed-off-by: Xenia Ragiadakou <burzalodowa@gmail.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>