net: dsa: don't print non-fatal MTU error if not supported

Commit 72579e14 ("net: dsa: don't fail to probe if we couldn't set
the MTU") changed, for some reason, the "err && err != -EOPNOTSUPP"
check into a simple "err". This causes the MTU warning to be printed
even for drivers that don't have the MTU operations implemented.
Fix that.

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>