net: tcp2: Access k_work in k_delayed_work using field name

Instead of casting k_delayed_work directly to k_work, use the
k_work field name. This avoids warnings from Coverity and
allows the code to work even if the k_delayed_work fields are
re-ordered in the future.

Coverity-CID: 214346
Fixes #28659

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>