mtd: move zero length verification to MTD API functions

In many places in drivers we verify for the zero length, but this is very
inconsistent across drivers. This is obviously the right thing to do, though.
This patch moves the check to the MTD API functions instead and removes a lot
of duplication.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Reviewed-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

drivers/mtd/chips/cfi_cmdset_0001.c

@@ -1566,9 +1566,6 @@ static int cfi_intelext_write_words (struct mtd_info *mtd, loff_t to , size_t le
 	int chipnum;
 	unsigned long ofs;
 
-	if (!len)
-		return 0;
-
 	chipnum = to >> cfi->chipshift;
 	ofs = to  - (chipnum << cfi->chipshift);