wl1271: Fix MAC address handling

This patch fixes MAC address handling. To achieve this, firmware booting had
to be delayed from the previous op_start to op_add_interface, which is the point
when the driver gets to know the configured MAC address. As the wl1271 only
supports one virtual interface, this even seems quite logical.

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Reviewed-by: Kalle Valo <kalle.valo@nokia.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>