net: pcs: xpcs: Explicitly return error on caps validation

If an unsupported interface is passed to the PCS validation callback there
is no need in further link-modes calculations since the resultant array
will be initialized with zeros which will be perceived by the phylink
subsystem as error anyway (see phylink_validate_mac_and_pcs()). Instead
let's explicitly return the -EINVAL error to inform the caller about the
unsupported interface as it's done in the rest of the pcs_validate
callbacks.

Signed-off-by: Serge Semin <fancer.lancer@gmail.com>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>