net/tipc/subscr.c: don't use ___constant_swab32

It's an internal implementation detail which we _should_ be free to change. 
So we did, and it promptly broke.

The compiler shold be able to work out when to use the __constant version
anyway.

Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>