rlcmac: rlc_window_dl: Fix RBB generation

We want to do mod sns on the BSN, not on the boolean returned by
gprs_rlcmac_rlc_v_n_is_received().
This is a typo which occurred while porting the code from osmo-pcu.git
void gprs_rlc_ul_window::update_rbb(char *rbb), where the following line
is used:
"""
	if (m_v_n.is_received((ssn()-1-i) & mod_sns()))
"""

Change-Id: I37c8fd5c2528f035f077c3e05105f913922ffd84
---
M src/rlcmac/rlc_window_dl.c
1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/src/rlcmac/rlc_window_dl.c b/src/rlcmac/rlc_window_dl.c
index a2bef5f..77153b6 100644
--- a/src/rlcmac/rlc_window_dl.c
+++ b/src/rlcmac/rlc_window_dl.c
@@ -163,7 +163,7 @@
 	unsigned int i;
 
 	for (i = 0; i < ws; i++) {
-		if (gprs_rlcmac_rlc_v_n_is_received(&dlw->v_n, ssn - 1 - i) & mod_sns)
+		if (gprs_rlcmac_rlc_v_n_is_received(&dlw->v_n, (ssn - 1 - i) & mod_sns))
 			rbb[ws - 1 - i] = 'R';
 		else
 			rbb[ws - 1 - i] = 'I';

