net: enetc: stop auto-configuring the port pMAC

The pMAC (ENETC_PFPMR_PMACE) is probably unconditionally enabled in the
enetc driver to allow RX of preemptible packets and not see them as
error frames. I don't know why TX preemption (ENETC_MMCSR_ME) is enabled
though. With no way to say which traffic classes are preemptible (all
are express by default), no preemptible frames would be transmitted
anyway.

Lastly, it may have been believed that the register write lock-step mode
(now deleted) needed the pMAC to be enabled at all times. I don't know
if that's true. However, I've checked that driver writes to PM1
registers do propagate through to the ENETC IP even when the pMAC is
disabled.

With such incomplete support for frame preemption, it's best to just
remove whatever exists right now and come with something more coherent
later.

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