net: phy: at803x: Replace of_gpio.h with what indeed is used

of_gpio.h in this driver is solely used as a proxy to other headers.
This is incorrect usage of the of_gpio.h. Replace it .h with what
indeed is used in the code.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>