Fix default numbers of iflib queue sets

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

Reviewed by: sbruno
Sponsored by: Limelight Networks
Differential Revision: https://reviews.freebsd.org/D13096