ipmi_si: fix unexpected driver unregister warning

If ipmi_si_platform_init()->platform_driver_register() fails,
platform_driver_unregister() called unconditionally will trigger
following warning,

ipmi_platform: Unable to register driver: -12
------------[ cut here ]------------
Unexpected driver unregister!
WARNING: CPU: 1 PID: 7210 at drivers/base/driver.c:193 driver_unregister+0x60/0x70 drivers/base/driver.c:193

Fix it by adding platform_registered variable, only unregister platform
driver when it is already successfully registered.

Reported-by: Hulk Robot <[e-mail]>
Signed-off-by: Kefeng Wang <[e-mail]>
Message-Id: <[e-mail]>

Signed-off-by: Corey Minyard <[e-mail]>

drivers/char/ipmi/ipmi_si_platform.c

···
 #include "ipmi_si.h"
 #include "ipmi_dmi.h"
 
+static bool platform_registered;
 static bool si_tryplatform = true;
 #ifdef CONFIG_ACPI
 static bool          si_tryacpi = true;
···
 	int rv = platform_driver_register(&ipmi_platform_driver);
 	if (rv)
 		pr_err("Unable to register driver: %d\n", rv);
+	else
+		platform_registered = true;
 }
 
 void ipmi_si_platform_shutdown(void)
 {
-	platform_driver_unregister(&ipmi_platform_driver);
+	if (platform_registered)
+		platform_driver_unregister(&ipmi_platform_driver);
 }