iio: magnetometer: si7210: simplify probe with devm_regulator_get_enable_read_voltage()

Simplify probe by using devm_regulator_get_enable_read_voltage() to
get, enable and read the regulator voltage in a single call, caching
the value at probe time.

This is a functional change as VDD voltage is now read once at probe
rather than dynamically on each temperature measurement. However, in
real deployments it is very rare for VDD to change after initial probe.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>