spi: stm32_spi: allow having xferlen equal to 0

The SPI API allows to have a 0 sized transfer length, in which case,
from an SPI interface point of view, only the CS signal get updated
if necessary.
Add a check in order to allow having xferlen equal to 0.

Change-Id: Ib0034eea1d5480022ec2e4cfc0f7131f15486805
Signed-off-by: Alain Volmat <alain.volmat@foss.st.com>
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/u-boot/+/523211
Domain-Review: Amelie DELAUNAY <amelie.delaunay@foss.st.com>
Reviewed-by: Alain VOLMAT <alain.volmat@st.com>
ACI: CIBUILD <MDG-smet-aci-builds@list.st.com>
ACI: CITOOLS <MDG-smet-aci-reviews@list.st.com>
Tested-by: Alain VOLMAT <alain.volmat@st.com>
Reviewed-by: Patrice CHOTARD <patrice.chotard@foss.st.com>

Author: Alain Volmat

drivers/spi/stm32_spi.c

@@ -445,6 +445,10 @@ static int stm32_spi_xfer(struct udevice *slave, unsigned int bitlen,
 	if (flags & SPI_XFER_BEGIN)
 		stm32_spi_set_cs(bus, slave_plat->cs, false);
 
+	/* If transfer length is 0, jump right to the end */
+	if (xferlen == 0)
+		goto done;
+
 	/* Be sure to have data in fifo before starting data transfer */
 	if (priv->tx_buf)
 		stm32_spi_write_txfifo(bus);
@@ -493,6 +497,7 @@ static int stm32_spi_xfer(struct udevice *slave, unsigned int bitlen,
 	setbits_le32(base + STM32_SPI_IFCR, SPI_IFCR_ALL);
 	stm32_spi_stopxfer(bus);
 
+done:
 	if (flags & SPI_XFER_END)
 		stm32_spi_set_cs(bus, slave_plat->cs, true);