net: macb: use phy_interface_mode_is_rgmii everywhere

There is one RGMII check not using the phy_interface_mode_is_rgmii()
helper. This prevents the driver from configuring the MAC properly when
using a phy-mode that is not just rgmii, e.g. rgmii-rxid. This became an
issue on sama5d3 xplained since the ksz9031 driver is hadling phy-mode
properly and the phy-mode has to be set to rgmii-rxid.

Fixes: bcf3440c

 ("net: phy: micrel: add phy-mode support for the KSZ9031 PHY")
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>