ath9k: parse the device configuration from an OF node

This allows setting the MAC address and specifying that the firmware
will be requested from userspace (because there might not be a hardware
EEPROM connected to the chip) for ath9k based PCI devices using
the device tree.

There is some out-of-tree code to "convert devicetree to
ath9k_platform_data" (for example in OpenWrt and LEDE) which becomes
obsolete with this patch.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>