media: atomisp: Make atomisp_set_raw_buffer_bitmap() static

atomisp_set_raw_buffer_bitmap() is only used in atomisp_cmd.c,
make it static. Unfortunately this still requires a forward
declaration (the function cannot be moved easily).

Still this will at least make it obvious to anyone reading the code
that the function is not used elsewhere.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>