net: dsa: print the MTU value that could not be set

These warnings become somewhat more informative when they include the
MTU value that could not be set and not just the errno.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Link: https://lore.kernel.org/r/20201205133944.10182-1-rasmus.villemoes@prevas.dk


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