USB: omninet: fix potential tty NULL dereference

Add check for return value of tty_port_tty_get,
since it can return NULL after port hangup that may happen anytime.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>