tools: iio: iio_generic_buffer: rename and change type of force variable

Replace the type of 'force' flag from int to bool and at the same time
rename it to 'force_autochannels' for better readability.

Suggested-by: Peter Meerwald-Stadler <pmeerw@pmeerw.net>
Signed-off-by: Eva Rachel Retuya <eraretuya@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>