drivers: wifi: esp: Convert driver to new DT device macros

Convert esp driver:

    NET_DEVICE_OFFLOAD_INIT -> NET_DEVICE_DT_INST_OFFLOAD_DEFINE

DT label is already required, so use it and drop CONFIG_WIFI_ESP_NAME
option.

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