brcm80211: fmac: register primary net device with device mac address

The primary net device was registered with a primary mac address and
upon IFUP it was set to match the actual mac address from the device.
This patch changes that and moves the brcmf_add_if() call to the common
part of the driver.

Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Franky (Zhenhui) Lin <frankyl@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>