USB: serial: ti_usb_3410_5052: fix NULL-deref at open

USB: serial: ti_usb_3410_5052: fix NULL-deref at open

commit ef079936d3cd09e63612834fe2698eeada0d8e3f upstream.

Fix NULL-pointer dereference in open() should a malicious device lack
the expected endpoints:

Unable to handle kernel NULL pointer dereference at virtual address 00000030
..
[<bf06a6b0>] (ti_open [ti_usb_3410_5052]) from [<bf02e118>] (serial_port_activate+0x68/0x98 [usbserial])

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Johan Hovold <johan@kernel.org>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>