mobile: fix -Wlogical-not-parentheses in gsm48_cc_init()

Found by clang:

  gsm48_cc.c:54:6: warning: logical not is only applied to the left
                   hand side of this comparison [-Wlogical-not-parentheses]
        if (!cc->mncc_upqueue.next == 0)
            ^                      ~~

Change-Id: Ic7ffd3aa25339e24a31bae1b7428f1f93e261858
---
M src/host/layer23/src/mobile/gsm48_cc.c
1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/src/host/layer23/src/mobile/gsm48_cc.c b/src/host/layer23/src/mobile/gsm48_cc.c
index 4db1925..43c93fc 100644
--- a/src/host/layer23/src/mobile/gsm48_cc.c
+++ b/src/host/layer23/src/mobile/gsm48_cc.c
@@ -52,7 +52,7 @@
 
 	cc->ms = ms;
 
-	if (!cc->mncc_upqueue.next == 0)
+	if (cc->mncc_upqueue.next != NULL)
 		return 0;
 
 	LOGP(DCC, LOGL_INFO, "init Call Control\n");

