ath10k: Update rx descriptor for WCN3990 target

WCN3990 rx descriptor uses different offset of msdu start, msdu end,
ppdu end, rx pkt end and rx frag info.
To accommodate different offsets, define respective fields in
rx descriptor of WCN3990 target.

Signed-off-by: Govind Singh <govinds@qti.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>