dsa: correctly determine the number of switches in a system

The number of connected switches was sourced from the number of
children to the DSA node, change it to the number of available
children, skipping any disabled switches.

Fixes: 5e95329b ("dsa: add device tree bindings to register DSA switches")
Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>