iwlwifi: remove TX_CMD id from transport layer

The transport layer doesn't need to know the TX_CMD id.
It can be set by the op_mode.
The transport layer still needs to know the layout of the
Tx command because of alignment issues and because of the
scratch pointer.

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