net: pcs: xpcs: fix error handling on failed to allocate memory

Drivers such as sja1105 and stmmac that call xpcs_create() expects an
error returned by the pcs-xpcs module, but this was not the case on
failed to allocate memory.

Fixed this by returning an -ENOMEM instead of a NULL pointer.

Fixes: 3ad1d171

 ("net: dsa: sja1105: migrate to xpcs for SGMII")
Signed-off-by: Wong Vee Khee <vee.khee.wong@linux.intel.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20210810085812.1808466-1-vee.khee.wong@linux.intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>