net: stmmac: xgmac: Disable the Timestamp interrupt by default

We don't use it anyway as XGMAC only supports polling for timestamp (in
current SW implementation). This greatly reduces the system load by
reducing the number of interrupts.

Fixes: 2142754f ("net: stmmac: Add MAC related callbacks for XGMAC2")
Signed-off-by: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>