USB: io_ti.c: remove unneeded null tty check

The Coverity checker (and Adrian Bunk) spotted an inconsistent NULL check of
port->tty (it's blindly dereferenced later without the check).

Alan Cox confirmed the check can go.

Signed-off-by: Ray Lee <ray-lk@madrabbit.org>
Cc: Adrian Bunk <bunk@kernel.org>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>