x86/platform/mellanox: Fix return value check in mlxplat_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 must therefor be replaced with IS_ERR().

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Vadim Pasternak <vadimp@mellanox.com>
Cc: platform-driver-x86@vger.kernel.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>