Fix bit masking logic in 'dead bus' return

In some cases (e.g. 16bit reads) a bad 'idle bus' return may be provided.
Promote the '1' to unsigned long prior to left-shifting it.

     1.1 diff -r d3bb6a6a04b7 -r 36367ebd34e0 src/memory.c
     1.2 --- a/src/memory.c	Sat Nov 17 21:28:48 2012 +0000
     1.3 +++ b/src/memory.c	Sat Nov 17 22:14:09 2012 +0000
     1.4 @@ -213,7 +213,7 @@
     1.5  			if (bits == 32)											\
     1.6  				return 0xFFFFFFFF;									\
     1.7  			else													\
     1.8 -				return (1 << bits)-1;								\
     1.9 +				return (1UL << bits)-1;								\
    1.10  		}															\
    1.11  	} while (0)
    1.12  /*}}}*/