net: stmmac: anarion: clean up interface parsing

anarion_config_dt() used a switch statement to check for the RGMII
modes, complete with an unnecessary "fallthrough", and also printed
the numerical value of the PHY interface mode on error. Clean this
up using the phy_interface_mode_is_rgmii() helper, and print the
English version of the PHY interface mode on error.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1u4FlV-000XjG-83@rmk-PC.armlinux.org.uk


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