iio: Unexport iio_scan_mask_set()

Individual drivers should not be messing with the scan mask that contains
the list of enabled channels. This is something that is supposed to be
managed by the core.

Now that the last few drivers that used it to configure a default scan mask
have been updated to not do this anymore we can unexport the function.

Note, this patch also requires moving a few functions around so they are all
declared before the first internal user.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Reviewed-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>