be2net: fix link failure after ethtool offline test

Certain cards in conjunction with certain switches need a little more
time for link setup that results in ethtool link test failure after
offline test. Patch adds a loop that waits for a link setup finish.

Changes in v2:
- added fixes header

Fixes: 4276e47e ("be2net: Add link test to list of ethtool self tests.")
Signed-off-by: Petr Oros <poros@redhat.com>
Reviewed-by: Ivan Vecera <ivecera@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>