selftests: pmtu.sh: fix result checking

In the PMTU test, when all previous tests are skipped and the new test
passes, the exit code is set to 0. However, the current check mistakenly
treats this as an assignment, causing the check to pass every time.

Consequently, regardless of how many tests have failed, if the latest test
passes, the PMTU test will report a pass.

Fixes: 2a9d3716 ("selftests: pmtu.sh: improve the test result processing")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Acked-by: Po-Hsu Lin <po-hsu.lin@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>