net: axienet: Handle deferred probe on clock properly

This driver is set up to use a clock mapping in the device tree if it is
present, but still work without one for backward compatibility. However,
if getting the clock returns -EPROBE_DEFER, then we need to abort and
return that error from our driver initialization so that the probe can
be retried later after the clock is set up.

Move clock initialization to earlier in the process so we do not waste as
much effort if the clock is not yet available. Switch to use
devm_clk_get_optional and abort initialization on any error reported.
Also enable the clock regardless of whether the controller is using an MDIO
bus, as the clock is required in any case.

Fixes: 09a0354c

 ("net: axienet: Use clock framework to get device clock rate")
Signed-off-by: Robert Hancock <robert.hancock@calian.com>
Signed-off-by: David S. Miller <davem@davemloft.net>