iwlwifi: adding MIMO3 support in rate scaling

Separated the MIMO tpt matrix into MIMO2 and MIMO3, adding MIMO3 support
in rate scaling algorithm. If the device support 3x3, then utilize all
three antenna (A/B/C) for tx to improve throughput.

Adding rs_switch_to_mimo3() function to allow switch to mimo3 modulation
mode from other modes(legacy/siso/mimo2). Adding rs_move_mimo3_to_other()
function to allow switch from mimo3 modulation mode to either siso or mimo2;
also support toggle between SGI and NGI.

Signed-off-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>