brcmfmac: remove use of unconditional access of struct wireless_dev::netdev

brcmfmac: remove use of unconditional access of struct wireless_dev::netdev

With the introduction of the P2P_DEVICE interface type an instance of
struct wireless_dev does not always have a netdev assigned to it. Better
use container_of() construct to obtain internal structure and go from
there.

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