tipc: relocate/coalesce node cast in tipc_named_node_up

Functions like this are called using unsigned longs from
function pointers.  In this case, the function is passed in
a node which is normally internally treated as a u32 by TIPC.

Rather than add more casts into this function in the future
for each added use of node within, move the cast to a single
place on a local.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>