Fix AD5421 driver bugs

AD5421 driver had a bug when integrating into a custom example:
- SPI R/W status checks check against an expected length, rather than
0/-1 which is what the No-OS API comments indicate it should return.
- Very minor: I added the "No-Op" code from AD5421 datasheet (0x09).
Currently the driver sends 0x00 which should suffice for reading data
back, but I added 0x09 to match what the AD5421 GUI sends.

Signed-off-by: Brandon Hurst <[email protected]>

drivers/dac/ad5421/ad5421.c

@@ -376,7 +376,7 @@ int32_t ad5421_set(struct ad5421_dev *dev,
 	status = no_os_spi_write_and_read(dev->spi_desc,
 					  tx_buffer,
 					  3);
-	if (status != 3) {
+	if (status != 0) {
 		return -1;
 	} else {
 		return 0;
@@ -393,13 +393,13 @@ int32_t ad5421_set(struct ad5421_dev *dev,
 int32_t ad5421_get(struct ad5421_dev *dev)
 {
 	int32_t return_val = 0;
-	uint8_t rx_buffer[3] = {0, 0, 0};
+	uint8_t rx_buffer[3] = {AD5421_NO_OP, 0, 0};
 	int8_t status = 0;
 
 	status = no_os_spi_write_and_read(dev->spi_desc,
 					  rx_buffer,
 					  3);
-	if (status != 3) {
+	if (status != 0) {
 		return -1;
 	}
 	return_val |= (int32_t)(rx_buffer[1] << 8);

drivers/dac/ad5421/ad5421.h

@@ -51,6 +51,7 @@
 #define AD5421_CMDWRGAIN        4
 #define AD5421_CMDRST			7
 #define AD5421_CMDMEASVTEMP     8
+#define AD5421_NO_OP			9
 #define AD5421_CMDRDDAC         129
 #define AD5421_CMDRDCTRL		130
 #define AD5421_CMDRDOFFSET      131