USB: serial: io_ti: fix NULL-deref at open

Fix NULL-pointer dereference when clearing halt at open should a
malicious device lack the expected endpoints when in download mode.

Unable to handle kernel NULL pointer dereference at virtual address 00000030
...
[<bf011ed8>] (edge_open [io_ti]) from [<bf000118>] (serial_port_activate+0x68/0x98 [usbserial])
[<bf000118>] (serial_port_activate [usbserial]) from [<c0470ca4>] (tty_port_open+0x9c/0xe8)
[<c0470ca4>] (tty_port_open) from [<bf000da0>] (serial_open+0x48/0x6c [usbserial])
[<bf000da0>] (serial_open [usbserial]) from [<c0469178>] (tty_open+0xcc/0x5cc)

Fixes: 1da177e4

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