soc: intel_adsp: fix debug window number

Fixes: commit 5f4b51f9 ("soc: intel_adsp: add a debug window slot
descriptor") - restore the maximum debug slot number. The added slot
resides in page 0, not at the end of the array. The bug didn't have
any run-time implications - no access to those high index value slots
is performed, all platforms so far have 3 slots at most, so this is a
pure correctness fix.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>