increase tx_queue size for RLC UM to match the max SN if 5bits are used

- This prevents some of the tests from blocking if too many SDUs are pushed
  down.

lib/src/upper/rlc_um.cc

@@ -31,7 +31,7 @@
 
 namespace srslte {
 
-rlc_um::rlc_um() : tx_sdu_queue(16)
+rlc_um::rlc_um() : tx_sdu_queue(32)
 {
   log = NULL;
   pdcp = NULL;