CVE-2023-54156 sfc: fix crash when reading stats while NIC is resetting

In the Linux kernel, the following vulnerability has been resolved: sfc: fix crash when reading stats while NIC is resetting efxnetstats .ndogetstats64 can be called during an ethtool selftest, during which time nicdata-mcstats is NULL as the NIC has been fini'd. In this case do not attempt to...