net: dsa: add dsa_is_unused_port helper

As the comment above the chunk states, the b53 driver attempts to
disable the unused ports. But using ds->enabled_port_mask is misleading,
because this mask reports in fact the user ports.

To avoid confusion and fix this, this patch introduces an explicit
dsa_is_unused_port helper which ensures the corresponding bit is not
masked in any of the switch port masks.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>