sctp: remove the typedef sctp_lower_cwnd_t

This patch is to remove the typedef sctp_lower_cwnd_t, and
replace with enum sctp_lower_cwnd in the places where it's
using this typedef.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>