fm10k: avoid possible null pointer dereference in fm10k_update_stats

It's currently possible for fm10k_update_stats to be called during the
window when we go down and the rings are removed. This can result in
a null pointer dereference. In fm10k_get_stats64 we work around this by
using ACCESS_ONCE and a null pointer check inside the loop. Use this
same flow in the fm10k_update_stats to avoid the potential null pointer.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Krishneil Singh <Krishneil.k.singh@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>