staging: octeon-ethernet: fix incorrect PHY mode

When connecting PHY, we set the mode to PHY_INTERFACE_MODE_GMII which is
not always correct. Specifically on boards where RGMII_RXID is needed
networking now longer works with at803x after commit 6d4cd041
("net: phy: at803x: disable delay only for RGMII mode").

Fix by passing the correct mode. Tested on EdgeRouter Lite
(RGMII_RXID, at803x PHY) and D-Link DSR-500N (RGMII, broadcom PHY).

Fixes: 6d4cd041

 ("net: phy: at803x: disable delay only for RGMII mode")
Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>