staging:iio:accel:sca3000 Drop custom ABI for watersheds.

For now we support only the 50% watershed interrupt and start and stop it
as part of the buffer bring up.  The 75% case may come back in future.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>