ath11k: rx path optimizations

During RX, accessing the reo dest ring descriptor directly is consuming
a lot of CPU cycles. Accessing the descriptor after copying it locally
has improved CPU usage by around ~10-15% while measuring throughput
in RX DBTC test cases(all radios are involved in the throughput
measurement).

HW tested: IPQ8074

Signed-off-by: Manikanta Pubbisetty <mpubbise@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1586421797-885-1-git-send-email-mpubbise@codeaurora.org