hwmon: (ads7871) Fix endianness bug in 16-bit register reads

The ads7871_read_reg16() function relies on spi_w8r16() to read the
16-bit sensor output. The ADS7871 device transmits the Least Significant
Byte (LSB) first.

On Little-Endian architectures, spi_w8r16() correctly reconstructs the
16-bit value. However, on Big-Endian architectures, the byte swapping
causes the first received byte (LSB) to be placed in the most significant
byte of the u16, resulting in corrupted voltage readings.

To fix this, cast the integer result of spi_w8r16() to a restricted
__le16 type and convert it to the host CPU's native byte order using
le16_to_cpu(). Negative error codes returned by the SPI core are caught
and returned prior to the conversion to avoid mangling the error status.

Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260418034601.90226-1-tabreztalks@gmail.com
Fixes: e0c70b8 ("hwmon: add TI ads7871 a/d converter driver")
Suggested-by: David Laight <david.laight.linux@gmail.com>
Signed-off-by: Tabrez Ahmed <tabreztalks@gmail.com>
Link: https://lore.kernel.org/r/20260502020844.110038-2-tabreztalks@gmail.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Author: Tabrez Ahmed

drivers/hwmon/ads7871.c

@@ -77,9 +77,13 @@ static int ads7871_read_reg8(struct spi_device *spi, int reg)
 static int ads7871_read_reg16(struct spi_device *spi, int reg)
 {
 	int ret;
+
 	reg = reg | INST_READ_BM | INST_16BIT_BM;
 	ret = spi_w8r16(spi, reg);
-	return ret;
+	if (ret < 0)
+		return ret;
+
+	return le16_to_cpu((__force __le16)ret);
 }
 
 static int ads7871_write_reg8(struct spi_device *spi, int reg, u8 val)