net: phy: ti: Fix dp83867 RGMII_TXID interface path

There is code that is specifically for RGMII_TXID interface, but this
will never get used because the code checks that the RGMII interface
is RGMII_ID to RGMII_RXID; RGMII_TXID is after this.

To fix this and avoid similar problems in the future, use the
phy_interface_is_rgmii helper function.

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