iwl4965: fix rxon flags set to wrong value for A mode in .erp_ie_changed

The patch fixes a bug that enables RXON_FLG_TGG_PROTECT_MSK erroneously
for A mode in the erp_ie_changed mac80211 callback.

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