pasemi_mac: workaround for erratum 5971

Implement workarounds for erratum 5971, where L2 hints aren't considered
properly unless the way hint is enabled on the interface. Since L2 isn't
setup to dedicate a way to headers, we need to reset the packet count
by hand so it won't run out of credits.
Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Jeff Garzik <jeff@garzik.org>