net: bcmgenet: correct return value of __bcmgenet_tx_reclaim

The reclaim function should return the number of buffer descriptors
reclaimed, not just the number corresponding to skb packets.

Also, remove the unnecessary computation when updating the consumer
index.

While this is not a functional problem it could degrade performance
of napi in a fragmented transmit stream.

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