qcacld-3.0: Fix data rate for mgmt tx packets in packet capture mode

Currently, data rate for management tx packets is filled only as
1 or 6 Mbps which is wrong.
Sniffer expect data rate multiple of 2. Hence fill the data rate with
multiple of 2 for tx management packets which received from firmware
and converting into mbps.

Change-Id: I5dd6515804cd3751925af0992a30a7441ebf9bbb
CRs-Fixed: 3444177