net: enetc: stop configuring pMAC in lockstep with eMAC

The MWLM bit (MAC write lock-step mode) allows register writes to the
pMAC to be auto-performed whenever the corresponding eMAC register is
written by the driver. This allows their configuration to remain
in sync.

The driver has set this bit since the initial commit, but it doesn't do
anything, since the hardware feature doesn't work (and the bit has been
removed from more recent versions of the documentation).

The driver does attempt, more or less, to keep those MAC registers in
sync by writing the same value once to e.g. ENETC_PM0_CMD_CFG (eMAC) and
once to ENETC_PM1_CMD_CFG (pMAC). Because the lockstep feature doesn't
work, that's what it will stick to.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>