ath5k doesn't support USB devices. Given that the RPi doesn't support PCI, PCIe or AHB (the three alternative bus systems ath5k compatible wlan cards can be connected to the host), it's no wonder that no according kernel modules are built for this target - it's impossible to connect them to an RPi.
If you're referring to ar5523, that module (and its firmware) doesn't support AP mode.
Do you know which module I need to install to support this dongle (even only in client mode, without AP mode)
I need it for Rpi Mod B. that use brcm2708/bcm2708 arch.
The driver to install would be (kmod-)ar5523, but this modules isn't packaged as part of the mac80211/ backports source package (it shouldn't be difficult to add, though).