i40e: add interrupt rate limit verbosity

Due to the resolution of the register controlling interrupt rate
limiting, setting certain values for the interrupt rate limit make it
appear as though the limiting is not completely accurate.  The problem
is that the interrupt rate limit is getting rounded down to the nearest
multiple of 4.  This patch fixes the problem by adding some feedback to
the user as to the actual interrupt rate limit being used when it
differs from the requested limit.  Without this patch setting interrupt
rate limits may appear to behave inaccurately.

Change-ID: I3093cf3f2d437d35a4c4f4bb5af5ce1b85ab21b7
Signed-off-by: Alan Brady <alan.brady@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>