iio: core: Fix IIO_VAL_FRACTIONAL calculation for negative values

iio: core: Fix IIO_VAL_FRACTIONAL calculation for negative values

Fixes IIO_VAL_FRACTIONAL for case when the result is negative and
exponent is 0.

example: if the result is -0.75, tmp0 will be 0 and tmp1 = 75
This causes the output to lose sign because of %d in snprintf
which works for tmp0 <= -1.

Reported-by: kernel test robot <lkp@intel.com> #error: uninitialized symbol tmp
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Anand Ashok Dumbre <anand.ashok.dumbre@xilinx.com>
Link: https://lore.kernel.org/r/1601910316-24111-1-git-send-email-anand.ashok.dumbre@xilinx.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>