myri10ge: make msi configurable at runtime through sysfs

Now that IRQ are requested is called on open() and freed on close(),
we can safely switch from/to MSI without unloading the module.

We are guaranteed to correctly free IRQ even if the sysfs file got
written in the meantime since the MSI initialization is stored in
mgp->msi_enabled.

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