net: phy: bcm7xx: add jumbo frame configuration to PHY

The BCM7XX PHY family requires special configuration to pass jumbo
frames. Do that during initial PHY setup.

Signed-off-by: Murali Krishna Policharla <murali.policharla@broadcom.com>
Reviewed-by: Scott Branden <scott.branden@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>