ath79-generic: enable ath10k-smallbuffers for low memory ath10k boards (#2699)

fixes OOM reboots due too limited ram with ath10k 5Ghz enabled
add some comments to describe the need for ath10k-ct replacement
tested stable on an TP-Link Archer C25v1
more details
https://github.com/openwrt/openwrt/commit/694757a08f620a9f24b70003542d9dcd0abeac46