isdn/gigaset: use USB API functions rather than constants

This patch introduces the use of the function usb_endpoint_num.

The Coccinelle semantic patch that makes these changes is as follows:

@@ struct usb_endpoint_descriptor *epd; @@

- (epd->bEndpointAddress & \(USB_ENDPOINT_NUMBER_MASK\|0x0f\))
+ usb_endpoint_num(epd)

Signed-off-by: Himangi Saraogi <himangi774@gmail.com>
Acked-by: Julia Lawall <julia.lawall@lip6.fr>
Acked-by: Tilman Schmidt <tilman@imap.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>