cxgb4: avoid accessing registers when clearing filters

In the Linux kernel, the following vulnerability has been resolved: cxgb4: avoid accessing registers when clearing filters Hardware register having the server TID base can contain invalid values when adapter is in bad state (for example, due to AER fatal error). Reading these invalid values in the register can lead to out-of-bound memory access. So, fix by using the saved server TID base when clearing filters.