net: ethernet: renesas: rcar_gen4_ptp: Fail on unknown register layout

Instead of printing a warning and proceeding with an unknown register
layout return an error. The only call site is already prepared to
propagate the error.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>