drivers: wifi: esp: process received packets in esp_rx thread

So far received packets were parsed (at AT command level) and allocated
in [esp_rx] thread. Then they were submitted to [esp_workq] thread for
processing (calling application callback).

This flow results in following deadlock when esp_workq thread waits on
response to some AT command:

  - [esp_rx] waits on allocation of new RX packet
  - [esp_workq] waits for [esp_rx] to process response to AT command
    that was just sent
  - blocked [esp_workq] prevents processing and deallocating RX packets
  - [esp_rx] times out on allocation and closes socket

Process RX packets directly from [esp_rx] thread to prevent above
deadlock.

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