pciback_field_is_dup needs a fix

[Jambunathan K <jambunathan@netxen.com>]

Shouldn't pciback_field_is_dup(dev, field->offset) be scoped to include 
the base_offset as well?

Currently with xen-3.0.4 the issue is this:

Adding of  PCI_PM_CTRL (at offset 4)  to the 'config_fields list' gets 
skipped because it ends up being a duplicate of PCI_COMMAND (at offset 4 
as well).  As a result when a PCI device behind a PCI frontend does a 
power up using pci_enable_device() the following message gets flashed on 
the console.

pciback 0000:0b:00.3: Driver tried to write to a read-only configuration 
space field at offset 0x84, size 2. This may be harmless, but if you 
have problems with your device:
1) see permissive attribute in sysfs
2) report problems to the xen-devel mailing list along with details of 
your device obtained from lspci.

Thanks,
Jambunathan K.