ixgbevf: Limit lowest interrupt rate for adaptive interrupt moderation to 12K

ixgbevf: Limit lowest interrupt rate for adaptive interrupt moderation to 12K

BugLink: http://bugs.launchpad.net/bugs/1536473
This patch is the ixgbevf version of commit 8ac34f10a5ea4 "ixgbe: Limit
lowest interrupt rate for adaptive interrupt moderation to 12K"

The same logic applies here as well as the same results since a netperf
test will starve for memory in the time from one Tx interrupt to the next.
As a result the ixgbevf driver underperformed when compared to vhost_net.

Signed-off-by: Alexander Duyck <aduyck@mirantis.com>
Tested-by: Darin Miller <darin.j.miller@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
(cherry picked from commit 8a9ca1104da0de6dd8551237e7d0e50eeeea4e80)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>