selftests: net: tcp_fastopen_backup_key.sh: fix shellcheck issue

When running tcp_fastopen_backup_key.sh the following issue was seen in
a busybox environment.
./tcp_fastopen_backup_key.sh: line 33: [: -ne: unary operator expected

Shellcheck showed the following issue.
$ shellcheck tools/testing/selftests/net/tcp_fastopen_backup_key.sh

In tools/testing/selftests/net/tcp_fastopen_backup_key.sh line 33:
        if [ $val -ne 0 ]; then
             ^-- SC2086: Double quote to prevent globbing and word splitting.

Rework to do a string comparison instead.

Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>