iio: buffer: extend short-hand use for 'indio_dev->buffer'

This change is both cosmetic and a prequel to adding support for attaching
multiple buffers per IIO device.

The IIO buffer sysfs attrs are mostly designed to support only one attached
buffer, and in order to support more, we need to centralize [in each attr
function] the buffer which is being accessed.

This also makes it a bit more uniform, as in some functions there is a
short-hand 'buffer' variable and at the same time the 'indio_dev->buffer'
is still access directly.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>