iwlwifi: unify tx_chains_num setting

There's no need to copy the same code for all
devices since none of the 5000 series devices
(that don't have the RX SISO override) don't
set the rx_with_siso_diversity variable.

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