brcm80211: util: use sk_buff_head in precedence queue functions

Instead of dealing with sk_buff prev pointers the queue functions
now make use of the sk_buff_head functions provided by the kernel.

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