net: stmmac: dwmac-sun8i: make clk really gated during rpm suspended

Currently, the dwmac-sun8i's stmmaceth clk isn't disabled even if the
the device has been runtime suspended. The reason is the driver gets
the "stmmaceth" clk as tx_clk and enabling it during probe. But
there's no other usage of tx_clk except preparing and enabling, so
we can remove tx_clk and its usage then rely on the common routine
stmmac_probe_config_dt() to prepare and enable the stmmaceth clk
during driver initialization, and benefit from the runtime pm feature
after probed.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>