staging: dgap: unwind on error in dgap_init_one()

The dgap_init_one() needs to handle error properly
if one of functions in dgap_init_one() is failed.

Introduce some functions for handling error in dgap_init_one()
 - dgap_tty_unregister() : unregister tty driver
 - dgap_free_flipbuf() : free flip buffer
 - dgap_release_remap() : release memory region and unmapped memory.

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