UBI: remove duplicate IO error codes

The 'UBI_IO_PEB_EMPTY' and 'UBI_IO_PEB_FREE' are essentially the same
and mean that there are only 0xFF bytes instead of headers. Simplify
UBI a little by turning them into a single 'UBI_IO_FF' error code.

Also, stop maintaining commentaries in 'ubi_io_read_vid_hdr()' which are
almost identical to commentaries in 'ubi_io_read_ec_hdr()'.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
diff --git a/drivers/mtd/ubi/scan.c b/drivers/mtd/ubi/scan.c
index 7e7c56d..37cb18f 100644
--- a/drivers/mtd/ubi/scan.c
+++ b/drivers/mtd/ubi/scan.c
@@ -748,7 +748,7 @@
 		return err;
 	else if (err == UBI_IO_BITFLIPS)
 		bitflips = 1;
-	else if (err == UBI_IO_PEB_EMPTY)
+	else if (err == UBI_IO_FF)
 		return add_to_list(si, pnum, UBI_SCAN_UNKNOWN_EC, &si->erase);
 	else if (err == UBI_IO_BAD_HDR_EBADMSG || err == UBI_IO_BAD_HDR) {
 		/*
@@ -817,7 +817,7 @@
 	else if (err == UBI_IO_BITFLIPS)
 		bitflips = 1;
 	else if (err == UBI_IO_BAD_HDR_EBADMSG || err == UBI_IO_BAD_HDR ||
-		 (err == UBI_IO_PEB_FREE && ec_corr)) {
+		 (err == UBI_IO_FF && ec_corr)) {
 		/* VID header is corrupted */
 		if (err == UBI_IO_BAD_HDR_EBADMSG ||
 		    ec_corr == UBI_IO_BAD_HDR_EBADMSG)
@@ -826,7 +826,7 @@
 		if (err)
 			return err;
 		goto adjust_mean_ec;
-	} else if (err == UBI_IO_PEB_FREE) {
+	} else if (err == UBI_IO_FF) {
 		/* No VID header - the physical eraseblock is free */
 		err = add_to_list(si, pnum, ec, &si->free);
 		if (err)