mac-learning: Fix inverted logic in is_learning_vlan().

When a bit is set in flood_vlans, that VLAN must be flooded, but the logic
here was reversed in the case where there were any flooded VLANs at all.
Thus, if a single VLAN was configured to be flooded, all VLANs except that
one were actually flooded.

The common case where no VLANs were to be flooded was handled correctly.

Reported-by: David Tsai <dtsai@nicira.com>