staging: dgap: change function names properly

dgap_do_remap() and dgap_release_remap() names could be
changed to dgap_remap() and dgap_unmap().

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