net: stmmac: rk: convert px30_set_rmii_speed() to .set_speed()

Convert px30_set_rmii_speed() to use the common .set_speed() method,
which eliminates another user of the older .set_*_speed() methods.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/E1uPk3J-004CFr-FE@rmk-PC.armlinux.org.uk


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