soc/intel_adsp: cavstool: minor error handling fixes

Don't "crash" when passing no argument at all.

Log exceptions, adding for instance the second line:

ERROR:cavs-fw:Could not map device in sysfs; run as root?
ERROR:cavs-fw:[Errno 13] Permission denied: \
             '/sys/bus/pci/devices/0000:00:0e.0/power/control'

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