kernel: mac80211: disable ath9k bands via device tree

These properties allow overriding the settings from the EEPROM
which indicate whether a band is enabled or not.
Setting this property is only needed when the RF circuit does not
support the 2.4GHz or 5GHz band while it is enabled nevertheless in the
EEPROM.

These patches will be replaced with a future upstream version which
will introduces an ieee80211 device tree property to disable bands.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>