mISDN: array underflow in open_bchannel()

There are two channels here.  User space starts counting channels at one
but in the kernel we start at zero.  If the user passes in a zero
channel that's invalid and could lead to memory corruption.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>