selftests: forwarding: lib: Split out setup_wait_dev()

Split out of setup_wait() a function setup_wait_dev() that waits for a
single device. This gives tests the opportunity to wait for a selected
device after they tinkered with its upness.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>