Rename this function to blk_get_device_part_str(). This is a better name
because it makes it clear that the function returns a block device and
parses a string.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Stephen Warren <swarren@nvidia.com>