ath10k: fix HTT service setup

The "disable credit flow" flag was set too late
and it never was in the HTC service request
message.

This patch prevents firmware from reporting
(useless) HTC credits for HTT service. HTT service
doesn't use nor need credits.

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