net: pse-pd: Use power limit at driver side instead of current limit

[ Upstream commit e0a5e2bba38aa61a900934b45d6e846e0a6d7524 ]

The regulator framework uses current limits, but the PSE standard and
known PSE controllers rely on power limits. Instead of converting
current to power within each driver, perform the conversion in the PSE
core. This avoids redundancy in driver implementation and aligns better
with the standard, simplifying driver development.

Remove at the same time the _pse_ethtool_get_status() function which is
not needed anymore.

Acked-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Stable-dep-of: f6093c5ec74d ("net: pse-pd: pd692x0: Fix power limit retrieval")
Signed-off-by: Sasha Levin <sashal@kernel.org>