ath10k: prefer kernel type 'u64' over 'u_int64_t'

Fixes checkpatch warnings:

drivers/net/wireless/ath/ath10k/htt.h:1477: Prefer kernel type 'u64' over 'u_int64_t'
drivers/net/wireless/ath/ath10k/htt.h:1480: Prefer kernel type 'u64' over 'u_int64_t'

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