staging: comedi: don't allow write() on async command set up for "read"

If a Comedi asynchronous command has been set up for data transfer in
the "read" direction on the current "write" subdevice (for those
subdevices that support both directions), don't allow the "write" file
operation as that would mess with the data in the comedi data buffer
that is written by the low-level comedi hardware driver.

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>