USB: serial: io_edgeport: fix slab-out-of-bounds read in edge_interrupt_callback

Fix slab-out-of-bounds read in the interrupt-URB completion handler.

The boundary condition should be (length - 1) as we access
data[position + 1].

Reported-and-tested-by:  <syzbot+37ba33391ad5f3935bbd@syzkaller.appspotmail.com>
Signed-off-by: Qiujun Huang <hqjagain@gmail.com>
Fixes: 1da177e4

 ("Linux-2.6.12-rc2")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Johan Hovold <johan@kernel.org>