brcmfmac: change mac address parameter in brcmf_add_if()

The function brcmf_add_if() is called with mac address set to NULL
for the primary interface. When handling IF ADD events the firmware
provides a address mask in the event to derive its mac address from
the primary mac address. Rename the parameter and use it as a mask.

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