USB: omninet: fix write_room

Fix regression introduced by commit 507ca9bc047666 ([PATCH] USB: add
ability for usb-serial drivers to determine if their write urb is
currently being used.) which inverted the logic in write_room so that it
returns zero when the write urb is actually free.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>