drivers: ethernet: lan865x: Avoid writing PLCA node count when nodeID not 0

The newest AN1760 application note - Revision F (DS60001760G - June 2024)
is recommending to not write the node count to PLCA_CTRL1 register when
the node is not the PLCA coordinator (i.e. its ID is not zero).

Signed-off-by: Lukasz Majewski <lukma@denx.de>