net: enetc: fix the incorrect clearing of IF_MODE bits

The enetc phylink .mac_config handler intends to clear the IFMODE field
(bits 1:0) of the PM0_IF_MODE register, but incorrectly clears all the
other fields instead.

For normal operation, the bug was inconsequential, due to the fact that
we write the PM0_IF_MODE register in two stages, first in
phylink .mac_config (which incorrectly cleared out a bunch of stuff),
then we update the speed and duplex to the correct values in
phylink .mac_link_up.

Judging by the code (not tested), it looks like maybe loopback mode was
broken, since this is one of the settings in PM0_IF_MODE which is
incorrectly cleared.

Fixes: c76a9721 ("net: enetc: force the RGMII speed and duplex instead of operating in inband mode")
Reported-by: Pavel Machek (CIP) <pavel@denx.de>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>