mtd: m25p80: add error handling to m25p80_read

index 4b5d7a4..634b0c4 100644
@@ -126,10 +126,11 @@ static int m25p80_read(struct spi_nor *nor, loff_t from, size_t len,
 	struct spi_device *spi = flash->spi;
 	struct spi_transfer t[2];
 	struct spi_message m;
-	unsigned int dummy = nor->read_dummy;
+	unsigned int cmd_len;
+	int ret;
 
 	/* convert the dummy cycles to the number of bytes */
-	dummy /= 8;
+	cmd_len = m25p_cmdsz(nor) + nor->read_dummy / 8;
 
 	spi_message_init(&m);
 	memset(t, 0, (sizeof t));

@@ -138,7 +139,7 @@ static int m25p80_read(struct spi_nor *nor, loff_t from, size_t len,
 	m25p_addr2cmd(nor, from, flash->command);
 
 	t[0].tx_buf = flash->command;
-	t[0].len = m25p_cmdsz(nor) + dummy;
+	t[0].len = cmd_len;
 	spi_message_add_tail(&t[0], &m);
 
 	t[1].rx_buf = buf;

@@ -146,10 +147,17 @@ static int m25p80_read(struct spi_nor *nor, loff_t from, size_t len,
 	t[1].len = len;
 	spi_message_add_tail(&t[1], &m);
 
-	spi_sync(spi, &m);
+	ret = spi_sync(spi, &m);
 
-	*retlen = m.actual_length - m25p_cmdsz(nor) - dummy;
-	return 0;
+	if (m.actual_length >= cmd_len)
+		*retlen = m.actual_length - cmd_len;
+	else
+		/* not even the command may have been
+		 * transferred completely
+		 */
+		*retlen = 0;
+
+	return ret;
 }
 
 static int m25p80_erase(struct spi_nor *nor, loff_t offset)