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.