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

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>