mobile: Fixed compiler warning at gsm322.c

This patch resolves the following issue:

In function 'memset',
    inlined from 'bargraph.constprop.14' at gsm322.c:325:8:
/usr/include/bits/string3.h:81:30: warning: call to '__warn_memset_zero_len' declared with attribute warning: memset used with constant zero length parameter; this could be due to transposed parameters [enabled by default]

src/host/layer23/src/mobile/gsm322.c

@@ -322,7 +322,8 @@ static char *bargraph(int value, int min, int max)
 	else
 		value -= min;
 
-	memset(bar, '=', value);
+	if (value)
+		memset(bar, '=', value);
 	bar[value] = '\0';
 
 	return bar;