drivers: wifi: esp: unify log level

2 (out of 3) components used CONFIG_WIFI_LOG_LEVEL, while the last one
didn't specify explicit log level. Always use CONFIG_WIFI_LOG_LEVEL in
all components.

While at it switch to LOG_MODULE_REGISTER(<module>, <log_level>), which
is a more compact way to define log level.

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