usb: common: ulpi: Add leading underscores for function name '__ulpi_register_driver()'

Fixes the following W=1 kernel build warning(s):

 drivers/usb/common/ulpi.c:151: warning: expecting prototype for ulpi_register_driver(). Prototype was for __ulpi_register_driver() instead

Cc: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-usb@vger.kernel.org
Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20210526130037.856068-4-lee.jones@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/common/ulpi.c

@@ -141,7 +141,7 @@ static const struct device_type ulpi_dev_type = {
 /* -------------------------------------------------------------------------- */
 
 /**
- * ulpi_register_driver - register a driver with the ULPI bus
+ * __ulpi_register_driver - register a driver with the ULPI bus
  * @drv: driver being registered
  * @module: ends up being THIS_MODULE
  *