iio: pressure: bmp280: read calibration data once

The calibration data is described as coming from an E2PROM and that
means it does not change. Just read it once at probe time and store
it in the device state container. Also toss the calibration data
into the entropy pool since it is device unique.

Reviewed-by: Vlad Dogaru <vlad.dogaru@intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>