net: stmmac: gmac5+: Fix missing return

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

Fixes: 7c728274 ("net: stmmac: gmac5+: Add support for Frame Preemption")
Signed-off-by: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>