iwlwifi: use pci registers defined in pci_regs.h

This patch replaces where possible usage of pci register
defined in the driver by ones defined in pci_regs.h

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>