net: dsa: print error on invalid port index

Looking for an -EINVAL all over the dsa code could take hours for
inexperienced DSA users.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20210106090915.21439-1-zajec5@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>