dsa: mv88e6xxx: fix debug print for SPEED_UNFORCED

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: Andrey Eremeev <Axtone4all@yandex.ru>
Fixes: 96a2b40c ("net: dsa: mv88e6xxx: add port's MAC speed setter")
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>