selftests: mptcp: simplify pm_nl_change_endpoint

This patch simplified pm_nl_change_endpoint(), using id-based address
lookups only. And dropped the fragile way of parsing 'addr' and 'id'
from the output of pm_nl_show_endpoints().

Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>