tools: iio: Add single-byte case for generic_buffer

Some sensors export data in an 8-bit format.
Add a single-byte case for the generic_buffer tool so that
these sensors' buffer data can be visualized.

Signed-off-by: Tiberiu Breana <tiberiu.a.breana@intel.com>
Reviewed-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>