pcu_l1_if: get rid of strange paging group calculation

The PCU uses a 16 bit integer value as paging group which does in fact
not really resamble the paging group. Instead it encodes the last three
digits of the IMSI from which the paging group can be calculated. The
value is then eventually converted back to Ascii-digits and then sent
over the PCU-IF interface. Lets get rid of this weird conversion and
use the IMSI directly.

Change-Id: I40d7fc14c9591b3de091e425faaf325421c70a0e
diff --git a/src/bts.cpp b/src/bts.cpp
index 504ba27..524961d 100644
--- a/src/bts.cpp
+++ b/src/bts.cpp
@@ -1131,7 +1131,7 @@
 		if (the_pcu->pcu_if_version >= 0x0b)
 			pcu_l1if_tx_pch_dt(bts, immediate_assignment, plen, tbf->imsi(), tbf->tlli());
 		else
-			pcu_l1if_tx_pch(bts, immediate_assignment, plen, ms_paging_group(tbf_ms(tbf)));
+			pcu_l1if_tx_pch(bts, immediate_assignment, plen, tbf->imsi());
 	}
 
 	bitvec_free(immediate_assignment);