shell: shell_adsp_memory_window: fix invalid cast in struct init

Fix invalid cast in SHELL_ADSP_MEMORY_WINDOW_DEFINE(). The type used in
the cast (struct shell_memwindow) refers to a non-existant type. As the
initialized struct field is of type "void *", compiler does not complain
about it, so there were no build warning/errors seen.

Reported-by: Jyri Sarha <jyri.sarha@linux.intel.com>
Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>