b44: use dev_kfree_skb() in b44_tx()

b44_tx() is run from softirq handler, it can use dev_kfree_skb() instead
of dev_kfree_skb_irq()

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