mptcp: token: fix unititialized variable

gcc complains about use of uninitialized 'num'. Fix it by doing the first
assignment of 'num' when the variable is declared.

Fixes: 96d890da

 ("mptcp: add msk interations helper")
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Link: https://lore.kernel.org/r/49e20da5d467a73414d4294a8bd35e2cb1befd49.1604308087.git.dcaratti@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>