mmc: use new hwpart API when CONFIG_BLK enabled

When CONFIG_BLK is enabled, the hwpart id is different with legacy
interface, update it to kame driver work with CONFIG_BLK.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>