Allow toggle of MTP debug

Via command line in TWRP:
twrp set tw_mtp_debug 1

This will enable debugging for MTP. You will need to disable and
re-enable MTP in order to see the debug logging.

Change-Id: Ia7bb0c584e10a4322b65ecf80a67ed7ee836b38e
diff --git a/mtp/twrpMtp.hpp b/mtp/twrpMtp.hpp
index 2dd56a2..bed81c0 100755
--- a/mtp/twrpMtp.hpp
+++ b/mtp/twrpMtp.hpp
@@ -33,7 +33,7 @@
 
 class twrpMtp {
 	public:
-		twrpMtp();
+		twrpMtp(int debug_enabled /* = 0 */);
 		pthread_t runserver(void);
 		void addStorage(std::string display, std::string path, int mtpid);
 	private: