ixgbe: fix possible NULL pointer deference in shutdown path

After freeing the rings we were not zeroing out the ring count values.
This patch now clears these counts correctly.

Reported-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Tested-by: Stephen Ko <stephen.s.ko@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>