myri10ge: fix management of the firmware 4KB boundary crossing restriction

Simpler way of dealing with the firmware 4KB boundary crossing
restriction for rx buffers.  This fixes a variety of memory
corruption issues when using an "uncommon" MTU with a 16KB
page size.

Signed-off-by: Brice Goglin <brice@myri.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>