platform/x86: mlx-platform: Fix copy-paste error in mlxplat_init()

The return value from platform_device_register_resndata() is not checked
correctly. The test is done against a wrong variable. This patch fix it.

Fixes: 0378123c ("platform/x86: mlx-platform: Add mlxreg-fan platform driver activation")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Vadim Pasternak <vadimp@mellanox.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>