cpuidle/pseries: Make symbol 'pseries_idle_driver' static

The sparse tool complains as follows:

drivers/cpuidle/cpuidle-pseries.c:25:23: warning:
 symbol 'pseries_idle_driver' was not declared. Should it be static?

'pseries_idle_driver' is not used outside of this file, so marks
it static.
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200714142424.66648-1-weiyongjun1@huawei.com

drivers/cpuidle/cpuidle-pseries.c

@@ -22,7 +22,7 @@
 #include <asm/idle.h>
 #include <asm/plpar_wrappers.h>
 
-struct cpuidle_driver pseries_idle_driver = {
+static struct cpuidle_driver pseries_idle_driver = {
 	.name             = "pseries_idle",
 	.owner            = THIS_MODULE,
 };