tools: iio: iio_generic_buffer: drop unneeded parentheses

Remove extra parentheses introduced in commit <73e176a tools: iio:
iio_generic_buffer: add -A to force-enable all channels>.

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>