ath10k: enable hw checksum for wcn3990

By default ath10k driver enables the support for HW_CHECKSUM
(NETIF_F_HW_CSUM). Since the TCP/UDP checksum calculation is not enabled
in the wcn3990 firmware the checksum is incorrect in the TCP/UDP packets
and all patckets are dropped. But due note that wcn3990 support in
ath10k is still incomplete so this isn't a critical fix (yet).

Enable hw checksum calculations in wcn3990 hardware by
setting the proper flags in msdu descriptor tso flags.

Signed-off-by: Rakesh Pillai <pillair@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>