ms_srs: add short_sn as config option for srsUE

this allows to change the announced support for PDCP
short sequence number lengths

Change-Id: Ibcf72dbb424ae3b6a87b60febb4dadf716407612
diff --git a/sysmocom/defaults.conf b/sysmocom/defaults.conf
index 8bbae42..8740d62 100644
--- a/sysmocom/defaults.conf
+++ b/sysmocom/defaults.conf
@@ -187,6 +187,7 @@
   log_all_level: warning
   log_nas_level: warning
   force_ul_amplitude: 0
+  nr_short_sn_support: true
 
 amarisoftue:
   tx_gain: 89