Fix errors in generated code when MAVLINK_CRC_EXTRA is disabled

Also ensures that packet length parameters use the appropriate *_LEN define
instead of hardcoding the raw values.

(strictly speaking I'm not using MAVLINK_CRC_EXTRA disabled, but the fix also let me replace the chunk of memory used by the MAVLINK_MESSAGE_CRCS array with a switch statement - since I'm only using 2 message types this was a big win... and then I moved from AVR to STM32 and it wasn't so critical anymore :)