iio: Fix two mpl3115 issues in measurement conversion

(i) pressure is 20-bit unsigned, not signed; the buffer description
is incorrect; for raw reads, this is just cosmetic

(ii) temperature is 12-bit signed, not 16-bit; this affects
readout of temperatures below zero as the sign bit is incorrectly
processed

reported via private mail

Cc: stable@vger.kernel.org
Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
Reported-by: Robert Deliën <robert@delien.nl>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>