iio: mlx96014: Replace offset sign

Changed the offset to negative as usual equation is: (raw +
offset)*scale and in this
case offset should be negative (as we deduct 273.15 Kelvin to get temperature
in Celsius).

Signed-off-by: Crt Mori <cmo@melexis.com>
Acked-by: Peter Meerwald <pmeerw@pmeerw.net>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>