brcmfmac: bail out of brcmf_txflowblock_if() for non-netdev interface

To avoid ending up in a NULL-pointer access, the function
brcmf_txflowblock_if() should only be called for interfaces
that have a netdev associated with it.

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