net: stmmac: make stmmac_interrupt() function more friendly to MSI

Refactor stmmac_interrupt() by introducing stmmac_common_interrupt()
so that we prepare the ISR operation to be friendly to MSI later.

Signed-off-by: Ong Boon Leong <boon.leong.ong@intel.com>
Signed-off-by: Voon Weifeng <weifeng.voon@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>