Fix serial string for FreeBSD.

I am sure we don't have /dev/ttyUSB*, and since I use /dev/cuau0 with a
cross-over cable to connect 2 machines and have a serial console, I guess
"/dev/cuau" is the SERIAL_STRING that's expected to work for me.  A quick test
would be cool however, but I don't own a serial NFC device.

libnfc/drivers/pn532_uart.c

@@ -54,8 +54,15 @@
     // MacOS
     // TODO: find UART connection string for PN53X device on Mac OS X
     #define SERIAL_STRING ""
+  #elif defined(__FreeBSD__)
+    // XXX: Not tested
+    #define SERIAL_STRING "/dev/cuau"
   #else
-    // *BSD, Linux and others POSIX systems
+    // Linux and maybe some operating systems
+    // FIXME: We'd rather have an #elif defined(__linux__) or something like
+    //        that and an #else that triggers an error at compile time instead
+    //        of "falling-back" on a value that is likely to not be suitable
+    //        for most operating systems.
     #define SERIAL_STRING "/dev/ttyUSB"
   #endif
 #endif