net: stmmac: Fix issues when number of Queues >= 4

When queues >= 4 we use different registers but we were not subtracting
the offset of 4. Fix this.

Found out by Coverity.

Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>