brcmfmac: use wiphy_read_of_freq_limits to respect limits from DT

This new helper reads extra frequency limits specified in DT and
disables unavailable chanels. This is useful for devices (like home
routers) with chipsets limited e.g. by board design.

In order to respect info read from DT we simply need to check for
IEEE80211_CHAN_DISABLED bit when constructing channel info.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>