iio: buffer: split buffer sysfs creation to take buffer as primary arg

iio: buffer: split buffer sysfs creation to take buffer as primary arg

Currently the iio_buffer_{alloc,free}_sysfs_and_mask() take 'indio_dev' as
primary argument. This change splits the main logic into a private function
that takes an IIO buffer as primary argument.

That way, the functions can be extended to configure the sysfs for multiple
buffers.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20200917125951.861-1-alexandru.ardelean@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>