staging: dgap: check unchecked return values

Check the return values of dgap_tty_register and
dgap_finalize_board_init. If they fail for a
particular board we should not be able to try
to use that board. IE. no device entries shall
be created for that board.

Signed-off-by: Mark Hounschell <markh@compro.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>