iwlwifi: remove command header flags field

The 'flags' field really has been reserved in the firmware API for a
very long time, probably since 4965. As a consequence, the field is
always 0 and checking for a IWL_CMD_FAILED_MSK flag makes no sense.

Rename the field to 'reserved', get rid of IWL_CMD_FAILED_MSK and
all the code for it.

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