net: pcs: lynx: check that the fwnode is available prior to use

Check that the fwnode is marked as available prior to trying to lookup
the PCS device, and return -ENODEV if unavailable. Document the return
codes from lynx_pcs_create_fwnode().

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