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 Deliƫn <robert@delien.nl> Signed-off-by: Jonathan Cameron <jic23@kernel.org>