ISDN: hfcsusb: checking idx of ep configuration

The syzbot test with random endpoint address which made the idx is
overflow in the table of endpoint configuations.

this adds the checking for fixing the error report from
syzbot

KASAN: stack-out-of-bounds Read in hfcsusb_probe [1]
The patch tested by syzbot [2]

Reported-by:  <syzbot+8750abbc3a46ef47d509@syzkaller.appspotmail.com>

[1]:
https://syzkaller.appspot.com/bug?id=30a04378dac680c5d521304a00a86156bb913522
[2]:
https://groups.google.com/d/msg/syzkaller-bugs/_6HBdge8F3E/OJn7wVNpBAAJ



Signed-off-by: Phong Tran <tranmanphong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>