iwlwifi: convert correctly rate_n_flags to PLCP index for mimo3 packets

The driver does not properly convert rate_n_flags to PLCP index for mimo3
packets. This makes mac80211 drop the packets

Signed-off-by: Daniel C. Halperin <daniel.c.halperin@intel.com>
Reviewed-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>