staging:iio:accel:sca3000 move to hybrid hard / soft buffer design.

In a similar fashion to other newer drivers (e.g. ti_am335x), instead
of using the hardware buffer support in IIO to directly access the hardware
fifo, insert a software fifo and feed that from the hardware one when
interrupts occur.  This gives a simpler structure to the data flows and
allows more flexibility over how often data is shipped to userspace etc.

This was also the only direct user of the simplistic generalization found
in ring_hw.h so that header is removed.

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