net: dsa: bcm_sf2: Forcibly configure IMP port for 1Gb/sec

We are still experiencing some packet loss with the existing advanced
congestion buffering (ACB) settings with the IMP port configured for
2Gb/sec, so revert to conservative link speeds that do not produce
packet loss until this is resolved.

Fixes: 8f1880cb ("net: dsa: bcm_sf2: Configure IMP port for 2Gb/sec")
Fixes: de34d708 ("net: dsa: bcm_sf2: Only 7278 supports 2Gb/sec IMP port")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>