tg3: Embrace pci_ioremap_bar()

Per Dave Miller's suggestion, replace the remaining ioremap_nocache()
call with pci_ioremap_bar().  Remove the two IORESOURCE_MEM checks as
they are redundant.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>