mtd: Update the function name to 'rfree'

Since commit 8d38a8459b0d ("mtd: Rename free() to rfree()")
the function has been renamed to rfree(), so update the description
inside the mtd_oob_region structure as well.

Fixes: 8d38a8459b0d ("mtd: Rename free() to rfree()")
Reported-by: Mikhail Kshevetskiy <mikhail.kshevetskiy@iopsys.eu>
Signed-off-by: Fabio Estevam <festevam@denx.de>
Acked-by: Michael Trimarchi <michael@amarulasolutions.com>
Reviewed-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
diff --git a/include/linux/mtd/mtd.h b/include/linux/mtd/mtd.h
index ff635bd..09f5269 100644
--- a/include/linux/mtd/mtd.h
+++ b/include/linux/mtd/mtd.h
@@ -122,7 +122,7 @@
  * @ecc: function returning an ECC region in the OOB area.
  *	 Should return -ERANGE if %section exceeds the total number of
  *	 ECC sections.
- * @free: function returning a free region in the OOB area.
+ * @rfree: function returning a free region in the OOB area.
  *	  Should return -ERANGE if %section exceeds the total number of
  *	  free sections.
  */