ath9k: OF: qca,disable-(2|5)ghz => ieee80211-freq-limit

ath9k: OF: qca,disable-(2|5)ghz => ieee80211-freq-limit

OpenWrt maintains two special out-of-tree DT properties:
"qca,disable-5ghz" and "qca,disable-2ghz". These are implemented
in a mac80211 ath9k patch "550-ath9k-disable-bands-via-dt.patch".

With the things being what they are, now might be a good
point to switch the devices to the generic and upstream
"ieee80211-freq-limit" property. This property is much
broader and works differently. Instead of disabling the
drivers logic which would adds the affected band and
channels. It now disables all channels which are not
within the specified frequency range.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>