r8169: use dev_kfree_skb() instead of dev_kfree_skb_irq()

rtl8169_tx_interrupt() is used from NAPI context, it can
directly free skbs. dev_kfree_skb_irq() is a leftover from
pre-NAPI times of this driver.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>