gsm: Fix wrong length in SI13 GPRS Cell Options IE

Older commit adding the 2 bits for Rel-4 extension forgot to increase
the length field (see TS 44.060 Table 12.24.1)

Fixes: 946bb95af1838fcf026587c51a6ad36e34e202c3
Change-Id: I20efb4403cdf6c5bc717502a7075630044142f17
diff --git a/src/gsm/gsm48_rest_octets.c b/src/gsm/gsm48_rest_octets.c
index b2b37b3..f45e818 100644
--- a/src/gsm/gsm48_rest_octets.c
+++ b/src/gsm/gsm48_rest_octets.c
@@ -911,12 +911,12 @@
 		bitvec_set_bit(bv, 1);
 		if (!gco->ext_info.egprs_supported) {
 			/* 6bit length of extension */
-			bitvec_set_uint(bv, (1 + 3)-1, 6);
+			bitvec_set_uint(bv, (1 + 5)-1, 6);
 			/* EGPRS supported in the cell */
 			bitvec_set_bit(bv, 0);
 		} else {
 			/* 6bit length of extension */
-			bitvec_set_uint(bv, (1 + 5 + 3)-1, 6);
+			bitvec_set_uint(bv, (1 + 5 + 5)-1, 6);
 			/* EGPRS supported in the cell */
 			bitvec_set_bit(bv, 1);