USB: serial: ti_usb_3410_5052: fix array underflow in completion handler

Similarly to a recently reported bug in io_ti, a malicious USB device
could set port_number to a negative value and we would underflow the
port array in the interrupt completion handler.

As these devices only have one or two ports, fix this by making sure we
only consider the seventh bit when determining the port number (and
ignore bits 0xb0 which are typically set to 0x30).

Fixes: 1da177e4

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