iwlwifi: set VHT beamformee STS cap correctly

It was set to zero which reflects support of a single STS.
Set according to the number of Rx antennas which correctly
reflects the number of STSs the STA can receive in a VHT NDP.
This improves beamforming and has been tested to improve Rx
throughput.

Signed-off-by: Eyal Shapira <eyal@wizery.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>