e1000e: pass pointer to ring struct instead of adapter struct

For ring-specific functions, pass a pointer to the ring struct instead of a
pointer to the adapter struct.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>