qed: Fix system crash in ll2 xmit

Cache number of fragments in the skb locally as in case
of linear skb (with zero fragments), tx completion
(or freeing of skb) may happen before driver tries
to get number of frgaments from the skb which could
lead to stale access to an already freed skb.

Signed-off-by: Manish Chopra <manishc@marvell.com>
Signed-off-by: Ariel Elior <aelior@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>