xen-pciback: Fix return in pm_ctrl_init()

[ Upstream commit 4745ea2628bb43a7ec34b71763b5a56407b33990 ]

Return NULL instead of passing to ERR_PTR while err is zero,
this fix smatch warnings:
drivers/xen/xen-pciback/conf_space_capability.c:163
 pm_ctrl_init() warn: passing zero to 'ERR_PTR'

Fixes: a92336a1 ("xen/pciback: Drop two backends, squash and cleanup some code.")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20211008074417.8260-1-yuehaibing@huawei.com


Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>