ixgbe: remove tie between NAPI work limits and interrupt moderation

As noted by Ben Hutchings and David Miller, work limits for NAPI
should not be tied to interrupt moderation parameters.  This
should be handled by NAPI, possibly through sysfs.

Neil Horman & Stephen Hemminger are working on a solution for
NAPI currently.  In the meantime, remove this tie between
work limits and interrupt moderation.

Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
CC: Ben Hutchings <bhutchings@solarflare.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>