driver: eth_esp32: fix shared clock check

Both MDIO and Ethernet drivers share the same clock subsystem.
After clock control update in #73807, clock_control_on()
now returns -EALREADY for already initialized clock subsystem.
As a result, ethernet driver won't initialize as needed.

Fixes #74440

Signed-off-by: Sylvio Alves <sylvio.alves@espressif.com>