iscsi-target: Fix hex2bin warn_unused compile message

Fix the following compile warning with hex2bin() usage:

drivers/target/iscsi/iscsi_target_auth.c: In function ‘chap_string_to_hex’:
drivers/target/iscsi/iscsi_target_auth.c:35: warning: ignoring return value of ‘hex2bin’, declared with attribute warn_unused_result
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/iscsi/iscsi_target_auth.c

@@ -30,9 +30,11 @@
 
 static int chap_string_to_hex(unsigned char *dst, unsigned char *src, int len)
 {
-	int j = DIV_ROUND_UP(len, 2);
+	int j = DIV_ROUND_UP(len, 2), rc;
 
-	hex2bin(dst, src, j);
+	rc = hex2bin(dst, src, j);
+	if (rc < 0)
+		pr_debug("CHAP string contains non hex digit symbols\n");
 
 	dst[j] = '\0';
 	return j;