staging: comedi: remain busy until read end-of-file

If a COMEDI subdevice is busy handling an asynchronous command in the
"read" direction, then after the command has terminated itself, the
"read" file operation handler, `comedi_read()` should keep the subdevice
busy until all available data has been read and it has returned 0 to
indicate an "end-of-file" condition.  Currently, it has a bug where it
can mark the subdevice as non-busy even when returning a non-zero count.
The bug is slightly hidden because the next "read" will return 0 because
the subdevice is no longer busy.  Fix it by checking the return count is
0 before deciding to mark the subdevice as non-busy.

The call to `comedi_is_subdevice_idle()` is superfluous as the
`become_nonbusy` variable will have been set to `true` when considering
becoming non-busy.  Strictly speaking, checking the return count is
superfluous too, as `become_nonbusy` doesn't get set to `true` unless
the count is 0, but check the return count anyway to make the intention
clearer.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>