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

Add the missing platform_device_unregister() before return
from mlxplat_init() in the error handling case.

Fixes: 6b266e91

 ("platform/x86: mlx-platform: Move regmap initialization before all drivers activation")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>