iwlwifi: mvm: Correctly set IGTK for AP

Currently when an IGTK is set for an AP, it is set as a regular key.
Since the cipher is set to CMAC, the STA_KEY_FLG_EXT flag is added to
the host command, which causes assert 0x253D on NICs that do not support
this.

Fixes: 85aeb58c ("iwlwifi: mvm: Enable security on new TX API")
Signed-off-by: Beni Lev <beni.lev@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>