staging: dgap: rename dgap_after_config_loaded() to dgap_alloc_flipbuf()

dgap_after_config_loaded() as function name doesn't tell
what it does.

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