ath10k: make target endianess more explicit

Some copy engine structures are target specific
and are uploaded to the device during
init/configuration.

This also cleans up a bit diag_mem_read/write
implicit byteswap mess leaving only
diag_access_read/write with an implicit endianess
byteswap.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>