staging: dgap: fix memory leak caused by double allocation of tty_structs

The tty_struct of serial_driver and print_driver were getting
allocated twice. One is allocated in tty_alloc_driver(), the other
is in dgap_tty_register(). So remove these in dgap_tty_register().

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