enb_srs: expose DL AWGN channel modifier for srsENB

we've added the modifier to configure Amarisoft eNB channel
model. This patch enables it for srsENB.

Change-Id: I30e65d0431b2d2792986128287caf8b23a22b2c1
diff --git a/src/osmo_gsm_tester/templates/srsenb.conf.tmpl b/src/osmo_gsm_tester/templates/srsenb.conf.tmpl
index ac94313..7409b3a 100644
--- a/src/osmo_gsm_tester/templates/srsenb.conf.tmpl
+++ b/src/osmo_gsm_tester/templates/srsenb.conf.tmpl
@@ -193,8 +193,16 @@
 # hst.fd_hz:         Doppler frequency in Hz
 # hst.init_time_s:   Initial time in seconds
 #####################################################################
+% if enb.enable_dl_awgn:
 [channel.dl]
-#enable        = false
+enable        = true
+% endif
+
+% if enb.enable_dl_awgn:
+[channel.dl.awgn]
+enable        = true
+snr           = ${enb.dl_awgn_snr}
+% endif
 
 [channel.dl.fading]
 #enable        = false