ixgbe: Move ring features into an enum, allowing easier future maintenance

From: Shannon Nelson <shannon.nelson@intel.com>

The ring_feature member of ixgbe_adapter is statically allocated based on
the supported features of the device.  When a new feature is added, we need
to manually update the static allocation.  This patch makes the feature
list an enum, eliminating the need for multiple updates to the code when
adding a new feature.

Signed-off-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>