fm10k: comment next_vf_mbx flow

Add a header comment explaining why we have the somewhat crazy mailbox
flow. This flow is necessary as it prevents the PF<->SM mailbox from
being flooded by the VF messages, which normally trigger a message to
the PF. This helps prevent the case where we see a PF mailbox timeout.

Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Acked-by: Matthew Vick <matthew.vick@intel.com>
Tested-by: Krishneil Singh <krishneil.k.singh@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>