net: phy: mscc: add missing shift for media operation mode selection

This patch adds a missing shift for the media operation mode selection.
This does not fix the driver as the current operation mode (copper) has
a value of 0, but this wouldn't work for other modes.

Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>