iwlwifi: change beamformee STS cap

All beamformee supporting chips have the ability to support
VHT NDP in up to 4 STSs. So change the published beamformee
STS cap accordingly to 3 as it should be Nsts-1.

Signed-off-by: Eyal Shapira <eyalx.shapira@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>