staging: dgap: introduce dgap_cleanup_nodes()

When a configration file is parsed with dgap_parsefile(),
makes nodes for saving configrations for board.

Making a node will allocate node memory and strings for saving
configrations with kstrdup().

So these are freed when dgap is unloaded or failed to initialize.

Signed-off-by: Daeseok Youn <daeseok.youn@gmail.com>
Tested-by: Mark Hounschell <markh@compro.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>