Fix default numbers of queue sets

[shurd]

Summary

The intent appears to be having one RX/TX queue set per core,
but since scctx->isc_n[tr]xqsets is set to max before calling
iflib_msix_init(), both end up being set to total number of cores.

Use ctx->ifc_sysctl_n[rt]xqs as the selected value and
scctx->isc_n[rt]xqsets as the max. This should result in what appears
to be the intended behaviour

Test Plan

Ensure default queues is number of cores across multiple
systems, and verify that override works as wxpected.