dm: blk: Rename get_dev() to blk_get_dev()

The current name is too generic. Add a 'blk_' prefix to aid searching and
make its purpose clearer.

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