Staging: comedi: COMEDI_BUFINFO with no async - report no bytes read or written

When the COMEDI_BUFINFO ioctl is used on a subdevice without
asynchronous streaming command support, set 'bytes_read = 0' and
'bytes_written = 0' in the buffer info returned back to the user.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>