core: Increase buffer size for sotware serial

- serial_mouse can lost mouse data when enabling debug print
- TODO: serial_soft blocks in ISR for too long time

tmk_core/protocol/serial_soft.c

@@ -86,7 +86,9 @@ void serial_init(void)
 }
 
 /* RX ring buffer */
-#define RBUF_SIZE   8
+#ifndef RBUF_SIZE
+#define RBUF_SIZE   256
+#endif
 static uint8_t rbuf[RBUF_SIZE];
 static uint8_t rbuf_head = 0;
 static uint8_t rbuf_tail = 0;