iio: lps331ap: Modify in_temp_scale calculation way

This patch modifies the way how the in_temp_scale output value is
calculated. With this implementation it is more clear how the value
is obtained.

Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>