nvme: Move block dev creation from uclass post_probe() to driver probe()

At present the block device creation happens in the NVMe uclass
driver post_probe() phase. In preparation to support multiple
namespaces, we should issue namespace identify before creating
block devices but that touches the underlying hardware hence it
is not appropriate to do such in the uclass driver post_probe().
Let's move it to driver probe() phase instead.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>