net: phy: vitesse: Fix cis8204 RGMII_ID code

Commit 79e86ccb "vitesse: remove duplicated
argument to ||" correctly removed a redundant check.

However, I believe that the original code was simply wrong, and should have
been checking against RGMII_ID.

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>