net: pcs: lynx: change lynx_pcs_create() to return error-pointers

Change lynx_pcs_create() to return an error-pointer on failure to
allocate memory, rather than returning NULL. This allows the removal
of the conversion in lynx_pcs_create_fwnode() and
lynx_pcs_create_mdiodev().

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