net: marvell: prestera: try to load previous fw version

Lets try to load previous fw version in case the latest one is missing on
existing system.

Signed-off-by: Vadym Kochan <vkochan@marvell.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>