perf tools: Omit unnecessary cast in perf_pmu__parse_scale

perf tools: Omit unnecessary cast in perf_pmu__parse_scale

There's no need to use a const char pointer, we can used char pointer
from the beginning and omit the unnecessary cast.

Reported-by: Ingo Molnar <[email protected]>
Signed-off-by: Jiri Olsa <[email protected]>
Cc: David Ahern <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>