cxgbe(4): Sanitize t4_num_vis during MOD_LOAD like all other t4_*
tunables. Add num_vis to the intrs_and_queues structure as it affects
the number of interrupts requested and queues created. In future
cfg_itype_and_nqueues might lower it incrementally instead of going
straight to 1 when enough interrupts aren't available.

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