staging: comedi: comedi_fops: send SIGIO according to command direction

`comedi_event()` is called from low-level drivers to handle comedi
asynchronous command event flags.  Some events cause waiting tasks to be
woken up, and a `SIGIO` signal to be sent via `kill_fasync()`.  The
signal code is `POLL_OUT` if the subdevice supports commands in the
"write" direction, or `POLL_IN` for the "read" direction.  If the
subdevice supports commands in either direction, it sends two `SIGIO`
signals, one with each code.  Change that latter case to only send one
`SIGIO` signal, using the direction of the current command to determine
the signal code.  If the `CMDF_WRITE` flag is set in the current
command, it's in the "write" direction, otherwise it's in the "read"
direction.

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>