ice: protect ring accesses with WRITE_ONCE

The READ_ONCE macro is used when reading rings prior to accessing the
statistics pointer. The corresponding WRITE_ONCE usage when allocating and
freeing the rings to ensure protected access was not in place. Introduce
this.

Signed-off-by: Ciara Loftus <ciara.loftus@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>