Read mac address from file

Due to the fact that this driver does not rely on an sdio or usb
driver as bootstrap, nor does the hardware have an eeprom we need to
work around the usual restraints for using read_firmware.

To be able to use read_firmware before we call
ieee80211_register_device we create a dummy device which is used to
read firmware from the system. This device is the disposed of and
normal init can continue.