ath10k: htc: simplified credit distribution

Simplified transmit credit distribution code somewhat.
Since the WMI control service will get assigned all credits
there is no need for having a credit_allocation array in
struct ath10k_htc.

Signed-off-by: Erik Stromdahl <erik.stromdahl@gmail.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>