myri10ge: add barrier in myri10ge_send_cmd

Add a barrier() in the usleep() loop in  myri10ge_send_cmd().
Without the barrier, some mips machine never notices that the
firmware has DMA'ed the response.

Signed-off-by: Brice Goglin <brice@myri.com>
Signed-off-by: Andrew Gallatin <gallatin@myri.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>