i40e: don't warn every time we clear an Rx timestamp register

The intent of this message was to indicate to a user that we might have
missed a timestamp event for a valid packet. The original method of
detecting the missed events relied on waiting until all 4 registers were
filled.

A recent commit d55458c0cd7a5 ("i40e: replace PTP Rx timestamp hang
logic") replaced this logic with much better detection
scheme that could detect a stalled Rx timestamp register even when other
registers were still functional.

The new logic means that a message will be displayed almost as soon as
a timestamp for a dropped frame occurs. This new logic highlights that
the hardware will attempt timestamp for frames which it later decides to
drop. The most prominent example is when a multicast PTP frame is
received on a multicast address that we are not subscribed to.

Because the hardware initiates the Rx timestamp as soon as possible, it
will latch an RXTIME register, but then drop the packet.

This results in users being confused by the message as they are not
expecting to see dropped timestamp messages unless their application
also indicates that timestamps were missing.

Resolve this by reducing the severity and frequency of the displayed
message. We now only print the message if 3 or 4 of the RXTIME registers
are stalled and get cleared within the same watchdog event. This ensures
that the common case does not constantly display the message.
Additionally, since the message is likely not as meaningful to most
users, reduce the message to a dev_dbg instead of a dev_warn.

Users can still get a count of the number of timestamps dropped by
reading the ethtool statistics value, if necessary.

Change-ID: I35494442226a444c418dfb4f91a3070d06c8435c
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>