pasemi_mac: fix bug in receive buffer dma mapping

pasemi_mac: fix bug in receive buffer dma mapping

skb->len isn't actually set to the size of the allocated skb, so don't
try to use it when figuring out how much to map.

(This hasn't surfaced as a real bug because we effectively disable
translation for the interface, but it still needs fixing for the future)

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