b44: use netdev_alloc_skb instead of dev_alloc_skb

The conversion in bf0dcbd9 missed the
new allocation in b44_rx.
This patch was used in OpenWRT for a long time.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: David S. Miller <davem@davemloft.net>