net: stmmac: reduce indentation when calling stmmac_xpcs_setup

There is no reason to embed an if within an if, we can just logically
AND the two conditions.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Wong Vee Khee <vee.khee.wong@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>