iwlwifi: do not set dual_stream_ant_msk for 3 streams

This patch fixes a bug introduced by commit "iwlwifi: adding MIMO3 support
in rate scaling". We should not set the dual_stream_ant_msk (for 2x2 MIMO)
when using a 3x3 rate, this will cause a SYSASSERT in uCode.

Note: there is no triple_stream_ant_msk, because all three antennas are used.

Signed-off-by: Daniel C Halperin <daniel.c.halperin@intel.com>
Acked-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>