net: fec: add variable reg_desc_active to speed things up

There is no need for complex macros every time we need to activate
a queue. Also, no need to call skb_get_queue_mapping when we already
know which queue it is using.

Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>
Signed-off-by: David S. Miller <davem@davemloft.net>