ipq40xx: use ath10k-smallbuffers for ZyXEL WRE6606 (#2842)

The WRE6066, has in contrast to other ip40xx devices, has only 128MB system RAM.
This results in OOM situations and instability, to circumvent this we need
to use ath10k-smallbuffers.

Signed-off-by: skorpy <skorpy@frankfurt.ccc.de>