mgb: cleanup

[kbowling]

Summary

Unsure if this will work as is but the correct function is for iflib to handle getting more work inside the interrupt filter and it uses the callbacks to mgb_{rx,tx}_queue_intr_{enable/disable}. This driver was doing everything in the admin interrupt.

[kbowling]

Enable the msix rxtx vec

[emaste]

With this change I can get a few pings to work but the driver stops working if I run iperf.

[kbowling]

will look at this once remote access is available