fix accelsim: ACC_READ cannot be 0 (it's used to check if a bit is set)

before that, accel data was all 0, but since the gyrosim also publishes
accel data, it was not a problem.

src/platforms/posix/drivers/accelsim/accelsim.cpp

@@ -75,7 +75,7 @@
 
 #define DIR_READ				(1<<7)
 #define DIR_WRITE				(0<<7)
-#define ACC_READ 				(0<<6)
+#define ACC_READ 				(1<<5)
 #define MAG_READ 				(1<<6)
 
 extern "C" { __EXPORT int accelsim_main(int argc, char *argv[]); }