iio: iio-utils: use channel modifier scaling if it exists

Now there are channel modifiers with their own scaling those should be
used when possible over the generic channel type scaling.

Examples are of IIO_TEMP channel having a generic scaling value, and
another having IIO_MOD_TEMP_AMBIENT modifier with another scaling value.

Previously the first scaling value for a channel type would be applied
to all channels of like type in iio_generic_buffer

Signed-off-by: Matt Ranostay <mranostay@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>