Fix swapped copyin(9) arguments in cxgb's iwch_arm_cq() function.
Detected by clang 3.7.0 with the warning:

sys/dev/cxgb/ulp/iw_cxgb/iw_cxgb_provider.c:309:18: error: variable
'rptr' is uninitialized when used here [-Werror,-Wuninitialized]

chp->cq.rptr = rptr;
               ^~~~

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