iio: st_pressure: Fix data sign

iio: st_pressure: Fix data sign

Datasheet of each device (lps331ap, lps25h, lps001wp, lps22hb) says that
the pressure and temperature data is a 2's complement.

I'm sending this the slow way, as negative pressures on these are pretty
unusual and the nature of the fixing of multiple device introduction patches
will make it hard to apply to older kernels - Jonathan.

Fixes: 217494e5b780 ("iio:pressure: Add STMicroelectronics pressures driver")
Fixes: 2f5effcbd097 ("iio: pressure-core: st: Expand and rename LPS331AP's channel descriptor")
Fixes: 7885a8ce6800 ("iio: pressure: st: Add support for new LPS001WP pressure sensor")
Fixes: e039e2f5b4da ("iio:st_pressure:initial lps22hb sensor support")
Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>