iwlwifi: fix check for a single rx antenna

valid_rx_ant is a bitmask of available antennas and not the number
of Rx antennas. Use num_of_ant and remove duplicate definitions
in both dvm and mvm.

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>