net: macb: ignore tx_clk if MII is used

If the MII interface is used, the PHY is the clock master, thus don't
set the clock rate. On Zynq-7000, this will prevent the following
warning:
  macb e000b000.ethernet eth0: unable to generate target frequency: 25000000 Hz

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20210120194303.28268-1-michael@walle.cc


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