ath10k: Use TX cksum offload only for CHECKSUM_PARTIAL

Otherwise ath10k will just checksum everything even if it did not
go through the TCP/IP stack (for example bridged frames). In the worst
case this could mean recreating the checksum for incorrect data.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>