iwlwifi: correctly size command string arrays

The command string array is assumed to be indexable by a full u8, so it
must have 256 entries (0-255), not just 255. A recent firmware change
(apparently) started using the command 0xff for a notification to the
host, causing the driver to crash in debug message/tracing code.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>