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/onenand/onenand_base.c

@@ -1753,9 +1753,6 @@ static int onenand_panic_write(struct mtd_info *mtd, loff_t to, size_t len,
 	pr_debug("%s: to = 0x%08x, len = %i\n", __func__, (unsigned int)to,
 			(int)len);
 
-	/* Initialize retlen, in case of early exit */
-	*retlen = 0;
-
 	/* Reject writes, which are not page aligned */
         if (unlikely(NOTALIGNED(to) || NOTALIGNED(len))) {
 		printk(KERN_ERR "%s: Attempt to write not page aligned data\n",