i40e: Fix sparse error: uninitialized symbol 'ring'

Init pointer with NULL in default switch case statement.

Previously the error was produced when compiling against sparse.
i40e_debugfs.c:582 i40e_dbg_dump_desc() error: uninitialized symbol 'ring'.

Fixes: 44ea803e ("i40e: introduce new dump desc XDP command")
Signed-off-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Signed-off-by: Arkadiusz Kubalewski <arkadiusz.kubalewski@intel.com>
Tested-by: Dave Switzer <david.switzer@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>