tipc: Minor optimization to deactivation of Ethernet media suppot

Change TIPC's shutdown code to deactivate generic networking support
before terminating Ethernet media support. The deactivation of generic
networking support causes all existing bearers to be destroyed, meaning
the Ethernet media termination routine no longer has to bother marking
them as unavailable.

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