iwlwifi: add option to test MFP

Add a Kconfig symbol to enable MFP for testing even
if the firmware file doesn't advertise it.

Signed-off-by: David Spinadel <david.spinadel@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>