staging: brcm80211: replaced PKTSETLEN macro by native __skb_trim call

- removed PKTSETLEN macro and used __skb_trim call instead.
- removed unused macros PKTSETPOOL and PKTPOOL

Reviewed-by: Henry Ptasinski <henryp@broadcom.com>
Reviewed-by: Brett Rudley <brudley@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>