ath10k-ct: reduce memory consumption

ath10k-ct uses a rather high number of buffers to communicate with the
QCA/Candelatech firmware. Especially the HTC (host-target-communication)
and HTT (host-target-transport) can take up a lot of memory when data is
transferred over a ath10k wifi link.

Even a 256MB device with three radios can go OOM (while sending to three
devices using UDP unicast/multicast packets) with the default buffer
limits. Unfortunately, this also reduced the maximum 5GHz throughput on an
IPQ40xx device from 494/432 Mbit/s for TCP transfers (download/upload) to
438/343 Mbit/s. These throughput values should still be good enough for a
Freifunk installation. And these changes are already used by ath10k in
OpenWrt 18.06.