tty: serial/crisv10.c: remove unnecessary null pointer check

The pointer tty is dereferened in line 3135, so it is not necessary to check
null again in line 3140.

Signed-off-by: Cong Ding <dinggnu@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>