staging:iio:accel:sca3000 Add write support to the low pass filter control

Also includes an available attribute.  The ordering of values appears
a bit random, but as the ABI doesn't specify this and we already have
both rising and falling lists I think this is fine.

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