brcmfmac: Ignore E_ADD_IF for ifidx 0.

All interfaces for ifidx is 0 are created ahead and therefor
double registrations for not only bssidx 0, but ifidx 0 should
be ignored in brcfm_add_if.

Reviewed-by: Arend Van Spriel <arend@broadcom.com>
Signed-off-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>