libertas_tf: fix skb tail pointer

skb->tail can't be meant here because it's not the same across 32/64 bit
compilations. This means there's no way the current driver can work on
64-bit architectures.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Cc: stable@kernel.org [2.6.27]
Signed-off-by: John W. Linville <linville@tuxdriver.com>