staging: comedi: don't allocate buffer space when polling for write

When handling the "poll" file operation and checking for `POLLOUT`,
don't allocate space from the buffer for writing, just check that space
is available for writing.  That check is done after checking that an
asynchronous "write" command is running on the subdevice.  Allocating
the buffer space before checking a "write" command is running can cause
problems if the subdevice supports commands in either direction and
currently has an active "read" command.

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>