iwlwifi: use rmb/wmb to protect indirect mmio operation

This patch protects iwlwifi indirect mmio operations with rmb() and
wmb(). It makes sure CPU reordering won't affect our indirect mmio
access.

Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>