xen/pciback: remove set but not used variable 'old_state'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/xen/xen-pciback/conf_space_capability.c: In function pm_ctrl_write:
drivers/xen/xen-pciback/conf_space_capability.c:119:25: warning:
 variable old_state set but not used [-Wunused-but-set-variable]

It is never used so can be removed.
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>

drivers/xen/xen-pciback/conf_space_capability.c

@@ -116,13 +116,12 @@ static int pm_ctrl_write(struct pci_dev *dev, int offset, u16 new_value,
 {
 	int err;
 	u16 old_value;
-	pci_power_t new_state, old_state;
+	pci_power_t new_state;
 
 	err = pci_read_config_word(dev, offset, &old_value);
 	if (err)
 		goto out;
 
-	old_state = (pci_power_t)(old_value & PCI_PM_CTRL_STATE_MASK);
 	new_state = (pci_power_t)(new_value & PCI_PM_CTRL_STATE_MASK);
 
 	new_value &= PM_OK_BITS;