PX4IO interface protocol: Fix rounding error

Issue: This conversion test failed with gcc 7.1.1 and clang 4.0.1:
```
ERROR [tests] conversion fail: input:  -0.8188, intermediate: -8187, result:  -0.8187
conv FAILED
Command 'tests' failed, returned -1
```

Fix: explicit rounding

src/modules/px4iofirmware/protocol.h

@@ -74,7 +74,7 @@
 #define SIGNED_TO_REG(_signed)	((uint16_t)(_signed))
 
 #define REG_TO_FLOAT(_reg)	((float)REG_TO_SIGNED(_reg) / 10000.0f)
-#define FLOAT_TO_REG(_float)	SIGNED_TO_REG((int16_t)((_float) * 10000.0f))
+#define FLOAT_TO_REG(_float)	SIGNED_TO_REG((int16_t)floorf((_float + 0.00005f) * 10000.0f))
 
 #define PX4IO_PROTOCOL_VERSION		4