soc/intel_adsp: cavstool: poll FW_STATUS even when --log-only

Failures to boot are not exclusive to the cavstool.py, they can happen
with the kernel driver and --log-only too. For such a situation this
commit adds a useful delay and these two log lines (before the mmap
crashes eventually):

 INFO:cavs-fw:Waiting for firmware handoff, FW_STATUS = 0x81000012
 WARNING:cavs-fw:Load failed?  FW_STATUS = 0x1006701

Signed-off-by: Marc Herbert <marc.herbert@intel.com>