myri10ge: increase and fix handoff timeout

Increase the handoff timeout to 512ms so as to give the aeluros based
NICs sufficient time to handoff without relying on the msleep() being
sloppy, and accidentally sleeping way longer than the 20ms we specified
in 20 separate 1ms sleeps.

Fix typo in the handoff sleep delay, which made it additive, not
exponential.

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