time: ntp: simplify the MAX_TICKADJ_SCALED definition

Impact: cleanup, no functionality changed

There's an ugly u64 typecase in the MAX_TICKADJ_SCALED definition,
this can be eliminated by making the MAX_TICKADJ constant's type
64-bit (signed).

kernel/time/ntp.o:

   text	   data	    bss	    dec	    hex	filename
   2504	    114	    136	   2754	    ac2	ntp.o.before
   2504	    114	    136	   2754	    ac2	ntp.o.after

md5:
   41f3009debc9b397d7394dd77d912f0a  ntp.o.before.asm
   41f3009debc9b397d7394dd77d912f0a  ntp.o.after.asm

Signed-off-by: Ingo Molnar <mingo@elte.hu>