iwlwifi: use writeb,writel,readl directly

That change will save us some CPU cycles at run time. Having port-based
I/O seems to be not possible for PCIe devices.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>