net: tulip: Constify tulip_tbl

It looks like all users of tulip_tbl are reads, so mark this table
as read-only.

$ git grep tulip_tbl  # edited to avoid line-wraps...
interrupt.c: iowrite32(tulip_tbl[tp->chip_id].valid_intrs, ...
interrupt.c: iowrite32(tulip_tbl[tp->chip_id].valid_intrs&~RxPollInt, ...
interrupt.c: iowrite32(tulip_tbl[tp->chip_id].valid_intrs, ...
interrupt.c: iowrite32(tulip_tbl[tp->chip_id].valid_intrs | TimerInt,
pnic.c:      iowrite32(tulip_tbl[tp->chip_id].valid_intrs, ioaddr + CSR7);
tulip.h:     extern struct tulip_chip_table tulip_tbl[];
tulip_core.c:struct tulip_chip_table tulip_tbl[] = {
tulip_core.c:iowrite32(tulip_tbl[tp->chip_id].valid_intrs, ioaddr + CSR5);
tulip_core.c:iowrite32(tulip_tbl[tp->chip_id].valid_intrs, ioaddr + CSR7);
tulip_core.c:setup_timer(&tp->timer, tulip_tbl[tp->chip_id].media_timer,
tulip_core.c:const char *chip_name = tulip_tbl[chip_idx].chip_name;
tulip_core.c:if (pci_resource_len (pdev, 0) < tulip_tbl[chip_idx].io_size)
tulip_core.c:ioaddr =  pci_iomap...