iio: mlx96014: Error checking from positive to negative

Dan Carpenter reported a static checker report and after his mail I
noticed that we actually return from function if positive value is
obtained from i2c read. This was remainder from when code was not in
separate function (which I changed during the review process).

Static checker reported
  drivers/iio/temperature/mlx90614.c:167
  mlx90614_iir_search()
    warn: this cast is a no-op
which meant that cast before negating is useless. Dan also proposed a
solution on nicer bit operation form.

Also changed magic number to macro in process as that was confusing.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Crt Mori <cmo@melexis.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>