net: fman_memac: use pcs-lynx's check for fwnode availability

Use pcs-lynx's check rather than our own when determining if the device
is available. This fixes a bug where the reference gained by
of_parse_phandle() is not dropped if the device is not available.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>