i2c: at91: fix SMBus quick command

The driver claims to support SMBus quick command but it was not the
case. This patch fixes this issue. Without it, i2cdetect finds imaginary
devices. And with some IP versions, trying to send 0 byte can cause
issue when writing data to an EEPROM.
Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>

[wsa: improved the commit message]
Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>

drivers/i2c/busses/i2c-at91.c

@@ -39,6 +39,7 @@
 #define	AT91_TWI_STOP		0x0002	/* Send a Stop Condition */
 #define	AT91_TWI_MSEN		0x0004	/* Master Transfer Enable */
 #define	AT91_TWI_SVDIS		0x0020	/* Slave Transfer Disable */
+#define	AT91_TWI_QUICK		0x0040	/* SMBus quick command */
 #define	AT91_TWI_SWRST		0x0080	/* Software Reset */
 
 #define	AT91_TWI_MMR		0x0004	/* Master Mode Register */
@@ -212,7 +213,11 @@ static int at91_do_twi_transfer(struct at91_twi_dev *dev)
 
 	INIT_COMPLETION(dev->cmd_complete);
 	dev->transfer_status = 0;
-	if (dev->msg->flags & I2C_M_RD) {
+
+	if (!dev->buf_len) {
+		at91_twi_write(dev, AT91_TWI_CR, AT91_TWI_QUICK);
+		at91_twi_write(dev, AT91_TWI_IER, AT91_TWI_TXCOMP);
+	} else if (dev->msg->flags & I2C_M_RD) {
 		unsigned start_flags = AT91_TWI_START;
 
 		if (at91_twi_read(dev, AT91_TWI_SR) & AT91_TWI_RXRDY) {