net: stmmac: xgmac: Fix missing return

If FPE is supposed to be disabled we need to return after disabling it.

Fixes: f0e56c8d ("net: stmmac: xgmac3+: Add support for Frame Preemption")
Signed-off-by: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>