ATHN_RIDX_MCS15
(ridx[0] <= ATHN_RIDX_MCS15) ? 0x3 : sc->txchainmask) |
(ridx[1] <= ATHN_RIDX_MCS15) ? 0x3 : sc->txchainmask) |
(ridx[2] <= ATHN_RIDX_MCS15) ? 0x3 : sc->txchainmask) |
(ridx[3] <= ATHN_RIDX_MCS15) ? 0x3 : sc->txchainmask);