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/lart.c

@@ -518,8 +518,6 @@ static int flash_write (struct mtd_info *mtd,loff_t to,size_t len,size_t *retlen
    printk (KERN_DEBUG "%s(to = 0x%.8x, len = %d)\n", __func__, (__u32)to, len);
 #endif
 
-   *retlen = 0;
-
    /* sanity checks */
    if (!len) return (0);