firmware: Google VPD: Fix return value check in vpd_platform_init()

In case of error, the function platform_device_register_simple()
returns ERR_PTR() and never returns NULL. The NULL test in the
return value check should be replaced with IS_ERR().

Fixes: 049a59db

 ("firmware: Google VPD sysfs driver")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>