spi: kirkwood: prevent limiting speed to 0

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 <[email protected]>
Tested-by: Kostya Porotchkin <[email protected]>
Reviewed-by: Marcin Wojtas <[email protected]>
Reviewed-by: Kostya Porotchkin <[email protected]>
Signed-off-by: Stefan Roese <[email protected]>