Bluetooth: host: Shrink size of struct bt_conn_tx
Bluetooth: host: Shrink size of struct bt_conn_tx

The `node` and `work` members are never used simultaneously.
Additionally k_work already has built-in support for being in a linked
list, however a union makes this change a bit cleaner.

Signed-off-by: Johan Hedberg <628991c7b0a19c384f9b99c21ec330ce952bb838@intel.com>

zephyrproject-rtos/zephyr,nashif/zephyr,finikorg/zephyr,Vudentz/zephyr,Vudentz/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,nashif/zephyr,nashif/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,Vudentz/zephyr,Vudentz/zephyr,nashif/zephyr,nashif/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr