net: Print num_rx_queues imbalance warning only when there are allocated queues

BugLink: http://bugs.launchpad.net/bugs/591416
There are a number of network drivers (bridge, bonding, etc) that are not yet
receive multi-queue enabled and use alloc_netdev(), so don't print a
num_rx_queues imbalance warning in that case.

Also, only print the warning once for those drivers that _are_ multi-queue
enabled.

Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>