net: dsa: Remove dynamic allocate of routing table

With a maximum of four switches, the size of the routing table is the
same as the pointer to it. Removing it makes the code simpler.

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