net: mdio_bus: Remove unused of_gpio.h

of_gpio.h is deprecated and subject to remove.
The driver doesn't use it, simply remove the unused header.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20240307122231.3677241-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>