drivers: wifi: esp: stop using pkt->work in TX path

Usage of k_work object from within net_pkt results in undefined behavior
in case when net_pkt is deallocated (by net_pkt_unref()) before work has
been finished.

Use per socket k_work object (sock->send_work) to submit send work and
put net_pkt objects onto k_fifo (sock->tx_fifo). Add a helper function
esp_socket_queue_tx() for that, which will make sure that packets are
enqueued only when send work handler will be successfully submitted (so
that all packets are consumed/dereferenced).

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>