drivers: isdn: use kernel macros to convert hex digit

Signed-off-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Karsten Keil <isdn@linux-pingi.de>
Cc: Tilman Schmidt <tilman@imap.cc>
Cc: netdev@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/isdn/hisax/q931.c b/drivers/isdn/hisax/q931.c
index 8b853d5..c0771f9 100644
--- a/drivers/isdn/hisax/q931.c
+++ b/drivers/isdn/hisax/q931.c
@@ -1152,20 +1152,11 @@
 {
 	register int i;
 	register char *t = txt;
-	register u_char w;
 
 	for (i = 0; i < cnt; i++) {
 		*t++ = ' ';
-		w = (p[i] >> 4) & 0x0f;
-		if (w < 10)
-			*t++ = '0' + w;
-		else
-			*t++ = 'A' - 10 + w;
-		w = p[i] & 0x0f;
-		if (w < 10)
-			*t++ = '0' + w;
-		else
-			*t++ = 'A' - 10 + w;
+		*t++ = hex_asc_hi(p[i]);
+		*t++ = hex_asc_lo(p[i]);
 	}
 	*t++ = 0;
 	return (t - txt);