libcommon: Fix log output for bts>0.

Fixes regression probably introduced in c696cc28.

For bts>0 logging doesn't show bts number correctly when printing lchan
identification string - it will always show it as "bts=0". The reason for
this is that the identification string is cached before bts->nr value is
set to a proper value.

This patch sets bts->nr as part of the first step of the bts structure
initialization, before caching happens thus making sure the cached
identification string is cached with the correct values.

Change-Id: I61c18a7f021fcb1ec00d34a745f4e3ab03416c2d
diff --git a/tests/channel/channel_test.ok b/tests/channel/channel_test.ok
index e4d625a..81d6569 100644
--- a/tests/channel/channel_test.ok
+++ b/tests/channel/channel_test.ok
@@ -1 +1,2 @@
 Testing subslot numbers for pchan types
+Testing the lchan printing: (bts=45,trx=0,ts=3,ss=4) (bts=45,trx=1,ts=3,ss=4)