staging: dgap: remove useless variable

dgap_major_serial_registered and dgap_major_transparent_print_registered
could be checked whether a board is initialized.
But it doesn't need to check that variables becasue dgap module
isn't calling the dgap_cleanup_tty() without initializing
for a board completely.

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