regulator: lp8788-ldo: Use ldo->en_pin to check if regulator is enabled by external pin

ldo->en_pin is set iff the regulator is enabled by external pin.

This patch sets ldo->en_pin to NULL if lp8788_gpio_request_ldo_en() fails, then
we can use it to determinate if the regulator is controlled by external pin or
register.

lp8788_get_ldo_enable_mode(), lp8788_ldo_ctrl_by_extern_pin() and
lp8788_ldo_is_enabled_by_extern_pin() functions are not used now, remove them.

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