[SCTP]: Remove an unused parameter from sctp_cmd_hb_timer_update

The 'asoc' parameter to sctp_cmd_hb_timer_update() is unused, and
we can remove it.

Signed-off-by: Gui Jianfeng <guijianfeng@cn.fujitsu.com>
Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>