mtd: remove retlen zeroing duplication

The MTD API function now zero the 'retlen' parameter before calling
the driver's method — do not do this again in drivers. This removes
duplicated '*retlen = 0' assignent from the following methods:

    'mtd_point()'
    'mtd_read()'
    'mtd_write()'
    'mtd_writev()'
    'mtd_panic_write()'

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

drivers/mtd/devices/mtd_dataflash.c

@@ -249,8 +249,6 @@ static int dataflash_read(struct mtd_info *mtd, loff_t from, size_t len,
 	pr_debug("%s: read 0x%x..0x%x\n", dev_name(&priv->spi->dev),
 			(unsigned)from, (unsigned)(from + len));
 
-	*retlen = 0;
-
 	/* Sanity checks */
 	if (!len)
 		return 0;
@@ -323,8 +321,6 @@ static int dataflash_write(struct mtd_info *mtd, loff_t to, size_t len,
 	pr_debug("%s: write 0x%x..0x%x\n",
 		dev_name(&spi->dev), (unsigned)to, (unsigned)(to + len));
 
-	*retlen = 0;
-
 	/* Sanity checks */
 	if (!len)
 		return 0;