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>

drivers/mtd/ubi/ubi.h

@@ -85,10 +85,7 @@
 /*
  * Error codes returned by the I/O sub-system.
  *
- * UBI_IO_PEB_EMPTY: the physical eraseblock is empty, i.e. it contains only
- *                   %0xFF bytes
- * UBI_IO_PEB_FREE: the physical eraseblock is free, i.e. it contains only a
- *                  valid erase counter header, and the rest are %0xFF bytes
+ * UBI_IO_FF: the read region of flash contains only 0xFFs
  * UBI_IO_BAD_HDR: the EC or VID header is corrupted (bad magic or CRC)
  * UBI_IO_BAD_HDR_EBADMSG: the same as %UBI_IO_BAD_HDR, but also there was a
  *                         data integrity error reported by the MTD driver
@@ -96,8 +93,7 @@
  * UBI_IO_BITFLIPS: bit-flips were detected and corrected
  */
 enum {
-	UBI_IO_PEB_EMPTY = 1,
-	UBI_IO_PEB_FREE,
+	UBI_IO_FF = 1,
 	UBI_IO_BAD_HDR,
 	UBI_IO_BAD_HDR_EBADMSG,
 	UBI_IO_BITFLIPS