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/chips/cfi_cmdset_0020.c

@@ -394,8 +394,6 @@ static int cfi_staa_read (struct mtd_info *mtd, loff_t from, size_t len, size_t
 	chipnum = (from >> cfi->chipshift);
 	ofs = from - (chipnum <<  cfi->chipshift);
 
-	*retlen = 0;
-
 	while (len) {
 		unsigned long thislen;
 
@@ -617,7 +615,6 @@ static int cfi_staa_write_buffers (struct mtd_info *mtd, loff_t to,
 	int chipnum;
 	unsigned long ofs;
 
-	*retlen = 0;
 	if (!len)
 		return 0;