net: dsa: vsc73xx: use macros for rgmii recognition

net: dsa: vsc73xx: use macros for rgmii recognition

It's preparation for future use. At this moment, the RGMII port is used
only for a connection to the MAC interface, but in the future, someone
could connect a PHY to it. Using the "phy_interface_mode_is_rgmii" macro
allows for the proper recognition of all RGMII modes.

Suggested-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://lore.kernel.org/r/20240417205048.3542839-4-paweldembicki@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>