staging: dgap: use dgap_release_remap() in dgap_cleanup_board()

Just simply use dgap_release_remap() in dgap_cleanup_board() for
releasing map memory.

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