fs/fat: Clean up open-coded sector <-> cluster conversions

Use the clust_to_sect() helper that was introduced earlier, and add an
inverse sect_to_clust(), plus update the various spots that open-coded
this conversion previously.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/fs/fat/fat.c b/fs/fat/fat.c
index 65873a2..f5f74c1 100644
--- a/fs/fat/fat.c
+++ b/fs/fat/fat.c
@@ -257,8 +257,7 @@
 	int ret;
 
 	if (clustnum > 0) {
-		startsect = mydata->data_begin +
-				clustnum * mydata->clust_size;
+		startsect = clust_to_sect(mydata, clustnum);
 	} else {
 		startsect = mydata->rootdir_sect;
 	}
@@ -594,9 +593,8 @@
 		mydata->data_begin = mydata->rootdir_sect +
 					mydata->rootdir_size -
 					(mydata->clust_size * 2);
-		mydata->root_cluster = (mydata->rootdir_sect -
-					mydata->data_begin) /
-					mydata->clust_size;
+		mydata->root_cluster =
+			sect_to_clust(mydata, mydata->rootdir_sect);
 	}
 
 	mydata->fatbufnum = -1;