mISDN: Bugfix hfcsusb: usb endpoint activation/deactivation

Here was a off by one in the activation/deactivation.
The additional activation in open_bchannel() did hide
it, but only if you do not try to use B2.

Signed-off-by: Martin Bachem <info@colognechip.com>
Signed-off-by: Karsten Keil <kkeil@linux-pingi.de>
Signed-off-by: David S. Miller <davem@davemloft.net>