Merge branch 'pcs-xpcs-cleanups'

Serge Semin says:

====================
net: pcs: xpcs: Cleanups before adding MMIO dev support

As stated in the subject this series is a short prequel before submitting
the main patches adding the memory-mapped DW XPCS support to the DW XPCS
and DW *MAC (STMMAC) drivers. Originally it was a part of the bigger
patchset (see the changelog v2 link below) but was detached to a
preparation set to shrink down the main series thus simplifying it'
review.

The patchset' content is straightforward: drop the redundant sentinel
entry and the header files; return EINVAL errno from the soft-reset method
and make sure that the interface validation method return EINVAL straight
away if the requested interface isn't supported by the XPCS device
instance. All of these changes are required to simplify the changes being
introduced a bit later in the framework of the memory-mapped DW XPCS
support patches.

Link: https://lore.kernel.org/netdev/20231205103559.9605-1-fancer.lancer@gmail.com
Changelog v2:
- Move the preparation patches to a separate series.
- Simplify the commit messages (@Russell, @Vladimir).
====================

Signed-off-by: David S. Miller <davem@davemloft.net>