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/sst25l.c

@@ -224,9 +224,6 @@ static int sst25l_read(struct mtd_info *mtd, loff_t from, size_t len,
 	if (len == 0)
 		return 0;
 
-	if (retlen)
-		*retlen = 0;
-
 	spi_message_init(&message);
 	memset(&transfer, 0, sizeof(transfer));