MIXERIOCLOADBUF ioctl: remove unnecessary string length restriction

mixer.cpp ensures the string is null-terminated (buffer length is 2048).

src/drivers/px4fmu/fmu.cpp

@@ -1470,7 +1470,7 @@ PX4FMU::pwm_ioctl(file *filp, int cmd, unsigned long arg)
 
 	case MIXERIOCLOADBUF: {
 			const char *buf = (const char *)arg;
-			unsigned buflen = strnlen(buf, 1024);
+			unsigned buflen = strlen(buf);
 			ret = _mixing_output.loadMixerThreadSafe(buf, buflen);
 			update_pwm_trims();