[SCTP]: Fix build warnings with IPV6 disabled.

Introduced by 270637abff0cdf848b910b9f96ad342e1da61c66
("[SCTP]: Fix a race between module load and protosw access")

Reported by Gabriel C:

In file included from net/sctp/sm_statetable.c:50:
include/net/sctp/sctp.h: In function 'sctp_v6_pf_init':
include/net/sctp/sctp.h:392: warning: 'return' with a value, in function returning void
In file included from net/sctp/sm_statefuns.c:62:
include/net/sctp/sctp.h: In function 'sctp_v6_pf_init':
include/net/sctp/sctp.h:392: warning: 'return' with a value, in function returning void
 ...

Signed-off-by: David S. Miller <davem@davemloft.net>