e1000e: convert driver to use extended descriptors

Some features currently not supported by the driver (e.g. RSS) require the
use of extended descriptors, but the driver is setup to only use legacy
descriptors in all modes except for when jumbo frames are enabled on some
parts.  Convert the driver to always use extended descriptors in order to
enable the forthcoming support of these other features.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Tested-by: Jeff Pieper <jeffrey.e.pieper@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>