iio: generic_buffer: Fix --trigger-num option

Initialize trig_num to -1 and handle trig_num=0 as a valid id.

Fixes: 7c7e9dad (iio: iio_generic_buffer: Add --trigger-num option)
Signed-off-by: Crestez Dan Leonard <leonard.crestez@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>