drivers: wifi: esp: always use net_pkt->work for sending

Currently there are two code paths when sending packets:
asynchronous (using workqueue) when zero timeout was specified and
synchronous in other cases. This doesn't seem to be justified, so
convert code to always schedule packet sending using workqueue.

Each net_pkt has an embedded work item, so use it instead of esp socket
specific work that was shared across all sent packets. This gives a
possibility to schedule multiple packets for sending.

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