net: wifi: Properly namespace public APIs

The twt and ps helper functions were not namespaced (were missing
wifi_ prefix).

Also change wifi_get_twt_... to more logical wifi_twt_get...
like the other twt prefixed functions and wifi_get_ps... to
wifi_ps_get... like the other ps prefixed functions.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>