iwlwifi: fix compile warning

Fixes following on big endian systems:
  CC [M]  drivers/net/wireless/iwlwifi/iwl-rx.o
  drivers/net/wireless/iwlwifi/iwl-rx.c: In function ‘iwl_rx_reply_rx’:
  drivers/net/wireless/iwlwifi/iwl-rx.c:1029: warning: integer overflow in
  expression

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Reported-by: Marcel Holtmann <marcel@holtmann.org>
Tested-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>