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

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: Jan Beulich <jbeulich@suse.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
(cherry picked from commit ee87d6d0d36d98c550f99274a81841033226e3bf)
Signed-off-by: Bob Liu <bob.liu@oracle.com>
Orabug: 24820937