brcmfmac: Fix rtnl_lock lockup when registering netdev.

When adding a virtual interface cfg80211 already has taken the
rtnl_lock. So when the netdevice is being registered the
register_netdevice should be used instead of register_netdev.

Reviewed-by: Arend Van Spriel <arend@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Piotr Haber <phaber@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>