iio: mlx90614: Check for errors in read values

The device uses the MSB of the returned temperature value as an error
flag.  Return a read error when this bit is set.

Signed-off-by: Vianney le Clément de Saint-Marcq <vianney.leclement@essensium.com>
Cc: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>