[paging] Use the number of different paging subchannels...

In our setup (1xCCCH combined, BS_AG_BLKS_RES=0,
BS_PA_MFRMS=0x3 -> 5) we have MAX(1,3-0) * 5 paging
sub-channels. Using this 15 I was able to successfully page
my phone/IMSI (934%15 -> 4).

My confusion is coming from the terms used for paging throughout
the documentation. GSM05.02 6.5.2 talks about "N = number of
paging blocks 'available' on one CCCH = (number of paging blocks
'available' in a 51-multiframe on one CCCH)xBS_PA_MFRMS" which
is already misguiding  and GSM04.08 is talking about number of
different paging subchannels on the CCCH and is providing a
formula.

I deduct that N == number of different paging subchannels on the CCCH
as of GSM04.08 and will simply test this with different IMSIs and
see if I can page them as well.
diff --git a/src/paging.c b/src/paging.c
index 0461572..29e97e5 100644
--- a/src/paging.c
+++ b/src/paging.c
@@ -58,7 +58,7 @@
 	ccch_conf = bts->chan_desc.ccch_conf;
 	bs_cc_chans = rsl_ccch_conf_to_bs_cc_chans(ccch_conf);
 	/* code word + 2, as 2 channels equals 0x0 */
-	blocks = bts->chan_desc.bs_pa_mfrms + 2;
+	blocks = rsl_number_of_paging_subchannels(bts);
 	group = get_paging_group(str_to_imsi(subscr->imsi),
 					bs_cc_chans, blocks);
 	return group;