tipc: remove unnecessary function pointers

Interaction between the functionality in server.c and subscr.c is
done via function pointers installed in struct server. This makes
the code harder to follow, and doesn't serve any obvious purpose.

Here, we replace the function pointers with direct function calls.

Acked-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>