Before activating a buffer make sure that at least one channel is enabled.
Activating a buffer with 0 channels enabled doesn't make too much sense and
disallowing this case makes sure that individual driver don't have to add
special case code to handle it.

Currently, without this patch enabling a buffer is possible and no error is
produced. With this patch -EINVAL is returned.

An example of execution with this patch and some instrumented print-code:
   root@analog:~# cd /sys/bus/iio/devices/iio\:device3/buffer
   root@analog:/sys/bus/iio/devices/iio:device3/buffer# echo 1 > enable
   0: iio_verify_update 748 indio_dev->masklength 2 *insert_buffer->scan_mask 00000000
   1: iio_verify_update 753
   2:__iio_update_buffers 1115 ret -22
   3: iio_buffer_store_enable 1241 ret -22
   -bash: echo: write error: Invalid argument
1, 2 & 3 are exit-error paths. 0 the first print in iio_verify_update()
rergardless of error path.

Without this patch (and same instrumented print-code):
   root@analog:~# cd /sys/bus/iio/devices/iio\:device3/buffer
   root@analog:/sys/bus/iio/devices/iio:device3/buffer# echo 1 > enable
   0: iio_verify_update 748 indio_dev->masklength 2 *insert_buffer->scan_mask 00000000
   root@analog:/sys/bus/iio/devices/iio:device3/buffer#
Buffer is enabled with no error.

Note from Jonathan: Probably not suitable for automatic application to stable.
This has been there from the very start.  It tidies up an odd corner
case but won't effect any 'real' users.

Fixes: 84b36ce5 ("staging:iio: Add support for multiple buffers")
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>