staging: dgap: introduce dgap_stop()

The dgap_init_module() need to unwind for cleanup variables properly.
Because dgap_init_module() calls dgap_cleanup_module() for freeing
variables but this function is possible to free variables
which are not allocated.

Signed-off-by: Daeseok Youn <daeseok.youn@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>