qcacld-3.0: Reuse preample type, vht_sig_a_1 and vht_sig_a_1

In monitor mode, current implementation
uses the preample type, vht_sig_a_1 and vht_sig_a_1
values associated with each mpdu, instead of reusing
the values from the first mpdu, to calculate data rates.
This is causing incorrect rates to be recorded in monitor mode logs

Reuse preample type, vht_sig_a_1 and vht_sig_a_1 of first
mpdu till the last mpdu is reached.

Change-Id: Ia6e5c1b3b0cc8d8b27f16cdfbd469fdba5c4a8f2
CRs-Fixed: 2276766