soc/intel_adsp: soc_adsp_halt_cpu always fails when NUM_CPUS <= 1

When the target has only a single CPU, this function cannot ever
succeed. Skip all of the drama and just return -EINVAL. This makes GCC 14
happy as it doesn't get confused about possible out of bounds access of the
soc_cpus_active array.

Signed-off-by: Keith Packard <keithp@keithp.com>