brcm80211: util: remove pointer traversal from brcmu_pkt_buf_free_skb

The function brcmu_pkt_buf_free_skb() was following the next pointer
to free all linked packets. However, it is only called with unlinked
packets so this can be removed.

Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Alwin Beukers <alwin@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>