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 <mingo@kernel.org>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20160308184230.GB7897@krava.redhat.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>