phy: marvell: a3700: Use (!ret) instead of (ret == 0)

In U-Boot it is usually written this way.

Signed-off-by: Marek Behun <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>