net: gem: ignore tx_clk if MII is used

net: gem: 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
error:
  zynq_gem ethernet@e000b000: failed to set tx clock rate 25000000

Signed-off-by: Martin Kaistra <martin.kaistra@linutronix.de>
Link: https://lore.kernel.org/r/20250415150400.136723-1-martin.kaistra@linutronix.de
Signed-off-by: Michal Simek <michal.simek@amd.com>