regulator: core: use regulator_ena_pin member

 The regulator_dev has regulator_enable_gpio structure.
 'ena_gpio' and 'ena_gpio_invert' were moved to in regulator_enable_gpio.

  regulator_dev   --->   regulator_enable_gpio
  .ena_gpio              .gpio
  .ena_gpio_invert       .ena_gpio_invert

  Pointer, 'ena_pin' is used for checking valid enable GPIO pin.

Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Reviewed-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>