ixgbevf: implement ethtool get/set coalesce

This patch adds support for ethtool's get_coalesce and set_coalesce command for
the ixgbevf driver. This enables dynamically updating the minimum time between
interrupts.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>