[PATCH] sky2: fix jumbo packet support

The truncate threshold calculation to prevent receiver from getting stuck
was incorrect, and it didn't take into account the upper limit on bits
in the register so the jumbo packet support was broken.

Signed-off-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>