iio: maxim_thermocouple: detect invalid storage size in read()

As found by gcc -Wmaybe-uninitialized, having a storage_bytes value other
than 2 or 4 will result in undefined behavior:

drivers/iio/temperature/maxim_thermocouple.c: In function 'maxim_thermocouple_read':
drivers/iio/temperature/maxim_thermocouple.c:141:5: error: 'ret' may be used uninitialized in this function [-Werror=maybe-uninitialized]

This probably cannot happen, but returning -EINVAL here is appropriate
and makes gcc happy and the code more robust.

Fixes: 231147e ("iio: maxim_thermocouple: Align 16 bit big endian value of raw reads")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/temperature/maxim_thermocouple.c

@@ -137,6 +137,8 @@ static int maxim_thermocouple_read(struct maxim_thermocouple_data *data,
 	case 4:
 		*val = be32_to_cpu(buf);
 		break;
+	default:
+		ret = -EINVAL;
 	}
 
 	/* check to be sure this is a valid reading */