drivers: wifi: esp: use context->send_cb and context->user_data

Callback and user data are saved in net_context structure. Those members
are used by native networking stack (net_if), so simply follow the same
pattern.

First of all this allows to reduce runtime information for driver socket
instance. Second and most important benefit is that it allows to move
send handling entirely to workqueue thread.

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