Merge branch 'enetc-mac-merge-prep'

Vladimir Oltean says:

====================
ENETC MAC Merge cleanup

This is a preparatory patch set for MAC Merge layer support in enetc via
ethtool. It does the following:

- consolidates a software lockstep register write procedure for the pMAC
- detects per-port frame preemption capability and only writes pMAC
  registers if a pMAC exists
- stops enabling the pMAC by default

Additionally, I noticed some build warnings in the driver which are new
in this kernel version, so patch 1/6 fixes those.
====================

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