iio:buffer: Simplify iio_buffer_is_active()

We can skip having to loop through all the device's buffers to see if a certain
buffer is active, if we let the buffer's list head point to itself when the
buffer is inactive.

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