rtlwifi: use tasklet_setup to initialize rx_work_tasklet

In commit d3ccc14d most of the tasklets in this driver was
updated to the new API. However for the rx_work_tasklet only the
type of the callback was changed from
  void _rtl_rx_work(unsigned long data)
to
  void _rtl_rx_work(struct tasklet_struct *t).

The initialization of rx_work_tasklet was still open-coded and the
function pointer just cast into the old type, and hence nothing sets
rx_work_tasklet.use_callback = true and the callback was still called as

  t->func(t->data);

with uninitialized/zero t->data.

Commit 6b8c7574 changed the casting of _rtl_rx_work a bit and
initialized t->data to a pointer to the tasklet cast to an unsigned
long.

This way calling t->func(t->data) might actually work through all the
casting, but it still doesn't update the code to use the new tasklet
API.

Let's use the new tasklet_setup to initialize rx_work_tasklet properly
and set rx_work_tasklet.use_callback = true so that the callback is
called as

  t->callback(t);

without all the casting.

Signed-off-by: Emil Renner Berthing <kernel@esmil.dk>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210126171550.3066-1-kernel@esmil.dk