igb: fix ethtool offline test

A bug was introduced with the following patch:

  Commmit bdbc0631
  Author: Eric Dumazet <eric.dumazet@gmail.com>
  igb: Add support for byte queue limits.

The ethtool offline tests will cause a perpetual link flap, this
is because the tests also need to account for byte queue limits (BQL).

CC: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: John Fastabend <john.r.fastabend@intel.com>
Tested-by: Jeff Pieper <jeffrey.e.pieper@intel.com>