net: dsa: Fix off-by-one in switch address parsing

cd->sw_addr is used as a MDIO bus address, which cannot exceed
PHY_MAX_ADDR (32), our check was off-by-one.

Fixes: 5e95329b

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