ath9k: support DT ieee80211-freq-limit property to limit channels

The common DT property can be used to limit the available channels
but ath9k has to manually call wiphy_read_of_freq_limits().

I would have put this into ath9k_of_init(). But it didn't work there.
The reason is that in ath9k_of_init() the channels and bands are not yet
registered in the wiphy struct. So there isn't any channel to flag as
disabled.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20211009212847.1781986-1-chunkeey@gmail.com