intel_th: msu: Fix unused variable warning on arm64 platform

Commit ba39bd83 ("intel_th: msu: Switch over to scatterlist")
introduced the following warnings on non-x86 architectures, as a result
of reordering the multi mode buffer allocation sequence:

> drivers/hwtracing/intel_th/msu.c: In function ‘msc_buffer_win_alloc’:
> drivers/hwtracing/intel_th/msu.c:783:21: warning: unused variable ‘i’
> [-Wunused-variable]
> int ret = -ENOMEM, i;
>                    ^
> drivers/hwtracing/intel_th/msu.c: In function ‘msc_buffer_win_free’:
> drivers/hwtracing/intel_th/msu.c:863:6: warning: unused variable ‘i’
> [-Wunused-variable]
> int i;
>     ^

Fix this compiler warning by factoring out set_memory sequences and making
them x86-only.

Suggested-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Shaokun Zhang <zhangshaokun@hisilicon.com>
Fixes: ba39bd83 ("intel_th: msu: Switch over to scatterlist")
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20190621161930.60785-2-alexander.shishkin@linux.intel.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>