tools: iio: iio_generic_buffer: add -A to force-enable all channels

If attribute/s is/are already enabled (by default or via scripts or
manual interaction), issuing -a will fail to enable the channels thereby
one has to manually disable the said attribute/s before proceeding with
auto-enabling.

Add a command-line option -A to force-activate all channels regardless
of their current state.

Suggested-by: Alison Schofield <amsfield22@gmail.com>
Signed-off-by: Eva Rachel Retuya <eraretuya@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>