net: stmmac: Correctly assing MAX MTU in XGMAC cores case

Maximum MTU for XGMAC cores is 16k thus the check for presence of XGMAC
shall be done first in order to assign correct value.

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