spi: kirkwood: prevent limiting speed to 0

After commit 1fe929ed497bcc8975be8d37383ebafd22b99dd2
("spi: kirkwood: prevent configuring speed exceeding max controller freq")
the spi frequency could be set to 0 on platform where spi-max-frequency
is not defined (e.g. on armada-388-gp). Prevent limiting speed in
mentioned cases.

Signed-off-by: Grzegorz Jaszczyk <jaz@semihalf.com>
Tested-by: Kostya Porotchkin <kostap@marvell.com>
Reviewed-by: Marcin Wojtas <marcin@marvell.com>
Reviewed-by: Kostya Porotchkin <kostap@marvell.com>
Signed-off-by: Stefan Roese <sr@denx.de>