selftests: mptcp: join: fix dev in check_endpoint

selftests: mptcp: join: fix dev in check_endpoint

There's a bug in pm_nl_check_endpoint(), 'dev' didn't be parsed correctly.
If calling it in the 2nd test of endpoint_tests() too, it fails with an
error like this:

 creation  [FAIL] expected '10.0.2.2 id 2 subflow dev dev' \
                     found '10.0.2.2 id 2 subflow dev ns2eth2'

The reason is '$2' should be set to 'dev', not '$1'. This patch fixes it.

Fixes: 69c6ce7b6eca ("selftests: mptcp: add implicit endpoint test case")
Cc: stable@vger.kernel.org
Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://lore.kernel.org/r/20240329-upstream-net-20240329-fallback-mib-v1-2-324a8981da48@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>