sgiioc4: kill duplicate ioremap()

By the time ide_dma_sgiioc4() gets called, sgiioc4_ide_setup_pci_device() will
have called ioremap() on the whole BAR0 region, so calling ioremap() on the DMA
registers means wasting a page. Replace this call by a mere address calculation,
based on the fact that IRQ registers (pointed to by 'hwif->io_ports.irq_addr')
are situated at offset 0 from BAR0.

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Cc: jeremy@sgi.com
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>