net: dsa: bcm_sf2: Only 7278 supports 2Gb/sec IMP port

The 7445 switch clocking profiles do not allow us to run the IMP port at
2Gb/sec in a way that it is reliable and consistent. Make sure that the
setting is only applied to the 7278 family.

Fixes: 8f1880cb ("net: dsa: bcm_sf2: Configure IMP port for 2Gb/sec")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>