tipc: Fix unsafe device list search when enabling bearer

Ensures that the device list lock is held while trying to locate
the Ethernet device used by a newly enabled bearer, so that the
addition or removal of a device does not cause problems.

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