Currently computation of Rx rate depends on Multi-user MCS
and NSS even for SU PPDU. For Single User PPDU, this becomes
invalid since computation of Rx rate should be based on SU MCS
and NSS.
To fix this, use respective PPDU information(MCS and NSS) and
calculate Rx rate based on ppdu_type.
Change-Id: Ic18cbabe192e2cafce5cdceb5644368ff6b7c9fb
CRs-fixed: 2617063