pasemi_mac: Fix reuse of free'd skb

Turns out we're freeing the skb when we detect CRC error, but we're
not clearing out info->skb. We could either clear it and have the stack
reallocate it, or just leave it and the rx ring refill code will reuse
the one that was allocated.

Reusing a freed skb obviously caused some nasty crashes of various kind,
as reported by Brent Baude and David Woodhouse.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Jeff Garzik <jeff@garzik.org>