tools: iio: privatize globals and functions in iio_generic_buffer.c file

[ Upstream commit ebe5112535b5cf389ca7d337cf6a0c1d885f9880 ]

Mostly a tidy-up.
But also helps to understand the limits of scope of these functions and
globals.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20210215104043.91251-24-alexandru.ardelean@analog.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Stable-dep-of: 2d3dff577dd0 ("tools: iio: iio_generic_buffer ensure alignment")
Signed-off-by: Sasha Levin <sashal@kernel.org>