net: stmmac: sti: use phy_interface_mode_is_rgmii()

Replace the custom IS_PHY_IF_MODE_RGMII() macro with our generic
phy_interface_mode_is_rgmii() inline function.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/E1u4jMd-000rCG-VU@rmk-PC.armlinux.org.uk


Signed-off-by: Jakub Kicinski <kuba@kernel.org>