staging: wfx: fix setting MAC address from DT

MAC address read from chip is unconditionally used even if a MAC
address is configured in device tree.

Reported-by: Marc Dorval <marc.dorval@silabs.com>
Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20191017093954.657-5-Jerome.Pouiller@silabs.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>