cxgbe(4): Fix compile warning for !RSS kernels

.../sys/dev/cxgbe/t4_main.c:7197:1: warning: unused function 'hashen_to_hashconfig' [-Wunused-function]
7197 | hashen_to_hashconfig(int hashen)

| ^~~~~~~~~~~~~~~~~~~~

1 warning generated.

Reported by: bz
Fixes: d381a6b4a552 cxgbe: use newly exposed RSS hash key API rather than ad-hoc hashing