staging: dgap: cleanup duplicated warning message on dgap_tty_init()

If true_count is not same with brd->nasync, warning messages are
printed. But it has duplicated messages within if statement.

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