ixgbe: Indicate support for pause frames in all cases

All the MACs supported by ixgbe support pause frames, so indicate
that support in ethtool. Also set advertising according to requested
mode.

Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>