net: enetc: catch negative return code from enetc_pf_to_port()

After the refactoring introduced in commit 87614b93 ("net: enetc:
create a common enetc_pf_to_port helper"), enetc_pf_to_port was coded up
to return -1 in case the passed PCIe device does not have a recognized
BDF.

Make sure the -1 value is checked by the callers, to appease static
checkers.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>