net: dsa: bcm_sf2: recalculate switch clock rate based on ports

Whenever a port gets enabled/disabled, recalcultate the required switch
clock rate to make sure it always gets set to the expected rate
targeting our switch use case. This is only done for the BCM7445 switch
as there is no clocking profile available for BCM7278.

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