Support 2400 baud

[tresf]

Reported downstream: qzind/tray#476.

https://github.com/java-native/jssc/blob/master/src/java/jssc/SerialPort.java#L49:L55

Baud 2400 is missing, despite it being supported natively.

Infact, the API doesn't seem to restrict the baud rate, it's jint baudRate and it's not an enumerated type:

JNIEXPORT jboolean JNICALL Java_jssc_SerialNativeInterface_setParams
  (JNIEnv *env, jobject object, jlong portHandle, jint baudRate, jint byteSize, jint stopBits, jint parity, jboolean setRTS, jboolean setDTR, jint flags){