iio: st_sensors: Call st_sensors_power_enable() from bus drivers

In case we would initialize two IIO devices from one physical device,
we shouldn't have a clash on regulators. That's why move
st_sensors_power_enable() call from core to bus drivers.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20210414195454.84183-4-andriy.shevchenko@linux.intel.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>