tipc: Minor correction to TIPC module unloading

Modifies TIPC's module unloading logic to switch itself into "single
node" mode before starting to terminate networking support. This helps
to ensure that no operations that require TIPC to be in "networking"
mode can initiate once unloading starts.

Signed-off-by: Allan Stephens <allan.stephens@windriver.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>