USB: serial: oti6858: fix NULL-deref at open

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

Unable to handle kernel NULL pointer dereference at virtual address 00000030
...
PC is at oti6858_open+0x30/0x1d0 [oti6858]

Note that a missing interrupt-in endpoint would have caused open() to
fail.

Fixes: 49cdee0e ("USB: oti6858 usb-serial driver (in Nokia CA-42
cable)")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Johan Hovold <johan@kernel.org>