IIO_UTILS: when attributes where read before they were ready, for example
           on the command line, sometime you could get:
analog@linaro:/sys/bus/iio/devices/iio:device5$ cat ./in_temp0_input
cat: ./in_temp0_input: Invalid argument
           In the iio_utils, this was a zero element, zero length string
           which gets passed up as success. (value of 0).
           This change causes the error value -EINVAL (Invalid argument)
           to be returned.

Signed-off-by: Robin Getz <robin.getz@analog.com>
FMCOMMS1: Sync DAC temp reading with the new iio_utils, and fix issues
          that causes temp to read 2.5C, rather than 25C.

Signed-off-by: Robin Getz <Robin.Getz@analog.com>