iio:pressure:ms5611: fix ms5607 temp compensation

Computation of sens2 was wrong and is fixed by this patch, sens2 should be:
    2 * (t - 2000)^2
See page 8 of ms5607 datasheet here:
    http://www.meas-spec.com/product/pressure/MS5607-02BA03.aspx



Signed-off-by: Gregor Boirie <gregor.boirie@parrot.com>
Acked-by: Lucas De Marchi <lucas.demarchi@intel.com>
Acked-by: Peter Meerwald-Stadler <pmeerw@pmeerw.net>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>