Debug print uses invalid check to detect if speed is unforced:
(speed != SPEED_UNFORCED) should be used instead of (!speed).

Found by Linux Verification Center (linuxtesting.org) with SVACE.
Signed-off-by: NAndrey Eremeev <Axtone4all@yandex.ru>
Fixes: 96a2b40c ("net: dsa: mv88e6xxx: add port's MAC speed setter")
Reviewed-by: NAndrew Lunn <andrew@lunn.ch>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>