net: systemport: Clear status to reduce spurious interrupts

Do something similar to commit d5810ca3 ("net: bcmgenet: clear
status to reduce spurious interrupts") and clear interrupts right before
servicing them. This reduces the number of interrupts by 10K
interrupts/sec for a TX TCP session 1Gbits/sec.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>