net: phy: marvell 88e151x: Fix handling of RGMII interface types

The 88E1518 code is programming the wrong registers for rgmii-id,
rgmii-txid and rgmii-rxid interfaces.

Since the PHY defaults to rgmii-id, it would appear that the code
was previously only used with sgmii and rgmii-id interfaces.

Tested on 88E1512 PHY in rgmii-id mode which is from the same family
as 88E1518.

Signed-off-by: Phil Edworthy <phil.edworthy@renesas.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>