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 <hulkci@huawei.com>
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Message-Id: <20190517101245.4341-1-wangkefeng.wang@huawei.com>

Signed-off-by: Corey Minyard <cminyard@mvista.com>