iio: pressure: bmp280: Make read calibration callback optional

iio: pressure: bmp280: Make read calibration callback optional

Newer models do not require read the calibration parameters and apply the
compensation algorithms in the sensor.

Suggested-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Angel Iglesias <ang.iglesiasg@gmail.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/bb1b95ab3f4e71d3c76543370325c5c9aaa07add.1676823250.git.ang.iglesiasg@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>