xen-pciback: short-circuit read path used for merging write values

There's no point calling xen_pcibk_config_read() here - all it'll do is
return whatever conf_space_read() returns for the field which was found
here (and which would be found there again). Also there's no point
clearing tmp_val before the call.
Signed-off-by: NJan Beulich <jbeulich@suse.com>
Signed-off-by: NDavid Vrabel <david.vrabel@citrix.com>

drivers/xen/xen-pciback/conf_space.c

@@ -230,10 +230,8 @@ int xen_pcibk_config_write(struct pci_dev *dev, int offset, int size, u32 value)
 		field_end = OFFSET(cfg_entry) + field->size;
 
 		 if (req_end > field_start && field_end > req_start) {
-			tmp_val = 0;
-
-			err = xen_pcibk_config_read(dev, field_start,
-						  field->size, &tmp_val);
+			err = conf_space_read(dev, cfg_entry, field_start,
+					      &tmp_val);
 			if (err)
 				break;