net: systemport: Simplify queue mapping logic

The use of a bitmap speeds up the finding of the first available queue
to which we could start establishing the mapping for, but we still have
to loop over all slave network devices to set them up. Simplify the
logic to have a single loop, and use the fact that a correctly
configured ring has inspect set to true. This will make things simpler
to unwind during device unregistration.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>