drivers/net/wan: fix sparse warning: make symbol static

Fix this sparse warning:

  drivers/net/wan/x25_asy.c:623:5: warning: symbol 'x25_asy_esc' was not declared. Should it be static?
Signed-off-by: NHannes Eder <hannes@hanneseder.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/wan/x25_asy.c

@@ -620,7 +620,7 @@ static struct net_device_stats *x25_asy_get_stats(struct net_device *dev)
   *			STANDARD X.25 ENCAPSULATION		  	 *
   ************************************************************************/
 
-int x25_asy_esc(unsigned char *s, unsigned char *d, int len)
+static int x25_asy_esc(unsigned char *s, unsigned char *d, int len)
 {
 	unsigned char *ptr = d;
 	unsigned char c;