Staging: comedi: usbdux.c: fix locking up of the driver when the comedi ringbuffer...

Staging: comedi: usbdux.c: fix locking up of the driver when the comedi ringbuffer runs empty

commit d4c3a56587af3edbe5f618b20e800e9f9fde13cb upstream.

Jan-Matthias Braun spotted a bug which locks up the driver when the
comedi ring buffer runs empty and provided a patch. The driver would
still send the data to comedi but the reader won't wake up any more.
What's required is setting the flag COMEDI_CB_BLOCK after new data has
arrived which wakes up the reader and therefore the read() command.

Signed-off-by: Bernd Porr <berndporr@f2s.com>
Signed-off-by: Leann Ogasawara <leann.ogasawara@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>