sky2: use pci_config access functions

Use the PCI layer config access functions. The driver was using the
memory mapped window in device, to workaround issues accessing the
advanced error reporting registers.

Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>
Signed-off-by: David S. Miller <davem@davemloft.net>