net: dsa: bcm_sf2: Fix IMP setup for port different than 8

Since it became possible for the DSA core to use a CPU port different
than 8, our bcm_sf2_imp_setup() function was broken because it assumes
that registers are applicable to port 8. In particular, the port's MAC
is going to stay disabled, so make sure we clear the RX_DIS and TX_DIS
bits if we are not configured for port 8.

Fixes: 9f91484f ("net: dsa: make "label" property optional for dsa2")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>