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

commit  16cd22819755c53b019d0abfa031b42bcf81e604 from
https://github.com/nxp-imx/linux-imx lf-6.1.y

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>
(cherry picked from commit 05b606b8

)
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Xiaolei Wang <xiaolei.wang@windriver.com>