Couldn't find any parameters in wpa_supplicant.conf to change the WMM priority. You could try the wpa_cli command
wmm_ac_addts <uplink/downlink/bidi> <tsid=0..7> <up=0..7> [nominal_msdu_size=#] [mean_data_rate=#] [min_phy_rate=#] [sba=#] [fixed_nominal_msdu]:To add WMM AC traffic stream.
The up(user priority) flag in it could be used to set the priority for the traffic stream added.
Other cli commands that you could try out are:
wmm_ac_delts <tsid>: delete WMM-AC traffic stream
wmm_ac_status : show status for Wireless Multi-Media Admission-Control