intel_th: msu: Split sgt array and pointer in multiwindow mode

intel_th: msu: Split sgt array and pointer in multiwindow mode

To allow the use of externally allocated SG tables further down the line,
change the code to reference the table via a pointer and make it point to
the locally allocated table by default.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20190627125152.54905-4-alexander.shishkin@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>