change PIIODevice read* and write* methods size to "ssize_t"

doc/examples/piiodevice.cpp

@@ -11,11 +11,11 @@ protected:
 		// open your device here
 		return if_success;
 	}
-	int readDevice(void * read_to, int max_size) override {
+	ssize_t readDevice(void * read_to, ssize_t max_size) override {
 		// read from your device here
 		return readed_bytes;
 	}
-	int writeDevice(const void * data, int max_size) override {
+	ssize_t writeDevice(const void * data, ssize_t max_size) override {
 		// write to your device here
 		return written_bytes;
 	}