ath10k: retrieve MAC address from system firmware if provided

Devices may provide their own MAC address via system firmware (e.g.,
device tree), especially in the case where the device doesn't have a
useful EEPROM on which to store its MAC address (e.g., for integrated
Wifi).

Use the generic device helper to retrieve the MAC address, and (if
present) honor it above the MAC address advertised by the card.

Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>