ath10k: initialize msdu ext. descriptor before use

ath10k: initialize msdu ext. descriptor before use

Initial QCA99X0 support has a known issue with TCP Tx throughput.
All other path such as UDP Tx/Rx and TCP Rx meet their expectation
(> 900Mbps), but TCP Tx marked as low as 5Mbps when single pair is
used on iperf.

The root cause is turned out because TSO flag is not initialized
properly so that firmware configures TSO in wrong way.
TSO flags in msdu extension descriptor is required to be reset
to indicate firmware there is no TSO is enabled, otherwise it
could act as TSO is enabled which causes huge throughput drop.

In fact, it's enough by resetting TSO flags only to prevent the
unexpected behavior, but initializing whole msdu ext. descriptor
will help to clear uncertainty of firmware could bring on as it
constantly updated.

Signed-off-by: Peter Oh <poh@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>