net: enetc: do not access non-existent registers on pseudo MAC

The ENETC4_PM_IEVENT and ENETC4_PM_CMD_CFG registers do not exist on the
ENETC pseudo MAC, so the driver should prevent from accessing them.

Fixes: 5175c1e4adca ("net: enetc: add basic support for the ENETC with pseudo MAC for i.MX94")
Signed-off-by: Wei Fang <wei.fang@nxp.com>
Tested-by: Claudiu Manoil <claudiu.manoil@nxp.com>
Reviewed-by: Claudiu Manoil <claudiu.manoil@nxp.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260324062121.2745033-4-wei.fang@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>