Bluetooth: tester: Fix using invalid elements with k_fifo

The documentation of k_fifo_put mentions that the items must have 32
bits reserved for kernel use as that is where the sys_snode_t pointer
is placed but this detail was not taken into consideration when
designing the cmd_queue which access each element as a btp_cmd.

Note: This probably used to work just because k_fifo_get used to place
data directly into thread's swap_data so the kernel never really touched
the list but with the changes to use k_poll the list is always updated
exposing this issue.

Jira: ZEP-2568

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>