brcmutil: simplify brcmu_pkt_free_skb()

The function brcmu_pkt_free_skb() use skb->destructor to decide
how the sk_buff should be freed. However, when running AP mode
with iptables configured this results in a kernel warning.

Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Piotr Haber <phaber@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>