ath6kl: Fix mapping uplink endpoint for AR6004

AR6004(UB134) firmware supports only LP Endpoint, So map
all Access Categories to Low Priority endpoints. This fixes a WPA2
connection issue as the uplink(tx) endpoint is appropriately
mapped in sync with the firmware.

Tested-by: Ben Gray <ben.r.gray@gmail.com>
Reported-by: Ben Gray <ben.r.gray@gmail.com>
Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>