netfilter: h323: Use mod_timer instead of set_expect_timeout

Simplify the code without any side effect. The set_expect_timeout is
used to modify the timer expired time.  It tries to delete timer, and
add it again.  So we could use mod_timer directly.

Signed-off-by: Gao Feng <fgao@ikuai8.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>