tools:iio:generic_buffer: sign-extend and shift data

Refactor process_scan() to handle signed and unsigned data, respect shifts
and the data mask for 2, 4 and 8 byte sized scan elements.

Signed-off-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>