wl1251: use skb_trim to make skb shorter

the current code is directly setting skb->len, which is not correct and
brings problems with HAVE_EFFICIENT_UNALIGNED_ACCESS enabled in config

Signed-off-by: Ivaylo Dimitrov <ivo.g.dimitrov.75@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>