staging:iio: Drop the unused buffer enable() and is_enabled() callbacks

Currently none of the buffer implementations implements the enable() or
is_enable() nor does core code try to call these. So it is safe to remove them.

Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>