srsenb_drb.conf.tmpl: decrease pollPDU for QCI9

due to internal PDU buffering between PDCP/RLC the poll frequency
needs to be increase to get quicker feedback.

Change-Id: I4114fb36e675b3f91c471558f716276ab24f1db5

src/osmo_gsm_tester/templates/srsenb_drb.conf.tmpl

@@ -78,7 +78,7 @@ qci_config = (
   rlc_config = {
     ul_am = {
       t_poll_retx = 120;
-      poll_pdu = 64;
+      poll_pdu = 8;
       poll_byte = 750;
       max_retx_thresh = 16;
     };