fm10k: add support for extra debug statistics

Add a private ethtool flag to enable display of these statistics, which
are generally less useful. However, sometimes it can be useful for
debugging purposes. The most useful portion is the ability to see what
the PF thinks the VF mailboxes look like.

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