drivers: wifi: esp_at: move socket allocation to cmd_ipd_parse_hdr()

This is just a step forward to make cmd_ipd_parse_hdr() capable of parsing
IP address and port when CONFIG_WIFI_ESP_AT_PASSIVE_MODE=n. After obtaining
this information (IP and port) it will be simply much easier to write this
information into 'struct esp_socket' directly, instead of passing to upper
layer.

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>