mptcp: Remove unnecessary test for __mptcp_init_sock()

__mptcp_init_sock() always returns 0 because mptcp_init_sock() used
to return the value directly.

But after commit 18b683bf

 ("mptcp: queue data for mptcp level
retransmission"), __mptcp_init_sock() need not return value anymore.

Let's remove the unnecessary test for __mptcp_init_sock() and make
it return void.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David S. Miller <davem@davemloft.net>