net: dsa: b53: add MTU configuration support

It looks like the Broadcom switches supported by the b53 driver don't
support precise configuration of the MTU, but just a mumbo-jumbo boolean
flag. Set that.

Also configure BCM583XX devices to send and receive jumbo frames when
ports are configured with 10/100 Mbps speed.

Signed-off-by: Murali Krishna Policharla <murali.policharla@broadcom.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>