ath10k: Refactor htt rx descriptor and remove last msdu check for WCN3990

WCN3990 target uses more fields in htt rx descriptor.
Refactor htt rx descriptor to not break legacy code.
Remove last msdu check in htt rx for WCN3990 which is not required.

CRs-Fixed: 2005906
Change-Id: Id2f670ecd72ea898a11a9d9eea61b5a1f2364d32
Signed-off-by: Rakesh Pillai <pillair@codeaurora.org>