iwlwifi: mvm: Set valid TX antennas value before calib request

We must set the valid TX antennas number in the ucode before
sending the phy_cfg_cmd and request for calibrations.

Signed-off-by: Dor Shaish <dor.shaish@intel.com>
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>