net: r6040: Reclaim transmitted buffers in NAPI

Instead of taking one interrupt per packet transmitted, re-use the same
NAPI context to free transmitted buffers. Since we are no longer in hard
IRQ context replace dev_kfree_skb_irq() by dev_kfree_skb().

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