brcm80211: util: move brcmu_pkttotlen() function to brcmfmac

The functions brcmu_pkttotlen() is only used in brcmfmac driver
so it has been moved there. It also does not use the sk_buff
next pointer anymore but walks a skb queue to determine the total
length.

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>