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/include/openbsc/gsm_data_shared.h b/include/openbsc/gsm_data_shared.h
index 0790807..60da2e5 100644
--- a/include/openbsc/gsm_data_shared.h
+++ b/include/openbsc/gsm_data_shared.h
@@ -899,7 +899,7 @@
 };
 
 
-struct gsm_bts *gsm_bts_alloc(void *talloc_ctx);
+struct gsm_bts *gsm_bts_alloc(void *talloc_ctx, uint8_t bts_num);
 struct gsm_bts *gsm_bts_num(struct gsm_network *net, int num);
 
 struct gsm_bts_trx *gsm_bts_trx_alloc(struct gsm_bts *bts);