staging: comedi: check command direction in poll() file operation

staging: comedi: check command direction in poll() file operation

`comedi_poll()` handles the poll() file operation for comedi devices.

If no asynchronous command has been set up on the current "read"
subdevice, it sets the `POLLIN` and `POLLRDNORM` bits in the return
value to indicate that the read() file operation would not block as it
would return an error.  Add a check so it also does that if the
asynchronous command has been set up in the "write" direction as that
also causes the read() file operation to return an error.

Similarly, if no asynchronous command has need set up on the current
"write" subdevice, it sets the `POLLOUT` and `POLLWRNORM` bits in the
return value to indicate that the write() file operation would not block
as it would return an error.  Add a check so it also does that if the
asynchronous command has been set up in the "read" direction as that
also causes the write() file operation to return an error.

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>