isdn: hisax: hfc_usb: use usb_fill_int_urb()

Using usb_fill_int_urb() helps to find code which initializes an
URB. A grep for members of the struct (like ->complete) reveal lots
of other things, too.

The `interval' parameter is now set differently on HS and SS. The
argument is fed from bInterval so it should be the right thing to do.

Cc: Karsten Keil <isdn@linux-pingi.de>
Cc: netdev@vger.kernel.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>