staging:iio: drop sw_ring buffer implementation.

Whilst this is IIO's oldest buffer implementation it is messy, poorly
implemented and whilst it works, no one is entirely sure it always will.

New IIO drivers have not been using this for some time and now all remaining
old users have been converted to use the kfifo based alternative.

Clearly a fifo isn't the same as a ring buffer but in many use cases it
really doesn't matter.  We also loose the watershed based poll implementation.
However having poll effectively report data only when the buffer was half
full was at best an 'unusual' use of the interface.

At somepoint in the future we may bring watersheds back on a different
buffer implementation, but then we will think a lot more about how to do
the interface first.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>