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

[ Upstream commit 09e088a4903bd0dd911b4f1732b250130cdaffed ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>