optimize ath_tx_findrix: there's no need to walk the rates table as sc_rixmap is an inverse map
optimize ath_tx_findrix: there's no need to walk the rates table as
sc_rixmap is an inverse map

NB: could eliminate the check for an invalid rate by filling in 0 for
    invalid entries but the rate control modules use it to identify
    bogus rates so leave it for now

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase