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/devices/mtd_dataflash.c

@@ -249,10 +249,6 @@ static int dataflash_read(struct mtd_info *mtd, loff_t from, size_t len,
 	pr_debug("%s: read 0x%x..0x%x\n", dev_name(&priv->spi->dev),
 			(unsigned)from, (unsigned)(from + len));
 
-	/* Sanity checks */
-	if (!len)
-		return 0;
-
 	/* Calculate flash page/byte address */
 	addr = (((unsigned)from / priv->page_size) << priv->page_offset)
 		+ ((unsigned)from % priv->page_size);
@@ -321,10 +317,6 @@ static int dataflash_write(struct mtd_info *mtd, loff_t to, size_t len,
 	pr_debug("%s: write 0x%x..0x%x\n",
 		dev_name(&spi->dev), (unsigned)to, (unsigned)(to + len));
 
-	/* Sanity checks */
-	if (!len)
-		return 0;
-
 	spi_message_init(&msg);
 
 	x[0].tx_buf = command = priv->command;
@@ -479,8 +471,6 @@ static ssize_t otp_read(struct spi_device *spi, unsigned base,
 
 	if ((off + len) > 64)
 		len = 64 - off;
-	if (len == 0)
-		return len;
 
 	spi_message_init(&m);