Set beacon interval value:
iwpriv ath0 bintval x
Replace x by the desired beacon interval in ms
Get beacon interval value:
iwpriv ath0 get_bintval
You'll get something like that:
That means the beacon interval value is 100ms. Beacon intervals close to the upper limit of 500 ms should be avoided when using wpa_supplicant. There appears to be a problem with repeated association and disassociation of the client with the AP.