dm: blk: Rename get_device() to blk_get_device_by_str()

The current name is too generic. The function returns a block device based
on a provided string. Rename it to aid searching and make its purpose
clearer. Also add a few comments.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Stephen Warren <swarren@nvidia.com>
diff --git a/cmd/usb_mass_storage.c b/cmd/usb_mass_storage.c
index 03b7e21..59bb4ee 100644
--- a/cmd/usb_mass_storage.c
+++ b/cmd/usb_mass_storage.c
@@ -69,7 +69,7 @@
 		if (!devnum)
 			break;
 
-		ret = get_device(devtype, devnum, &block_dev);
+		ret = blk_get_device_by_str(devtype, devnum, &block_dev);
 		if (ret < 0)
 			goto cleanup;