iio:buffer: Free active scan mask in iio_disable_all_buffers()

Usually the active scan mask is freed in __iio_update_buffers() when the buffer
is disabled. But when the device is still sampling when it is removed we'll end
up disabling the buffers in iio_disable_all_buffers(). So we also need to free
the active scan mask here, otherwise it will be leaked.

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