iwlwifi: Added support for 3 antennas

Added support for 3 antennas for Legacy, SISO and MIMO2.
MIMO3 is still not supported yet.

Signed-off-by: Guy Cohen <guy.cohen@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>