net: stmmac: rk: replace empty set_to_rmii() with supports_rmii

net: stmmac: rk: replace empty set_to_rmii() with supports_rmii

Rather than providing a now-empty set_to_rmii() method to indicate
that RMII is supported, switch to setting ops->supports_rmii instead.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Tested-by: Heiko Stuebner <heiko@sntech.de> #px30,rk3328,rk3568,rk3588
Link: https://patch.msgid.link/E1vnYxq-00000007hor-3yXt@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>