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

Fix NULL-pointer dereference at open should the device lack a bulk-in or
bulk-out endpoint:

Unable to handle kernel NULL pointer dereference at virtual address 00000030
...
PC is at iuu_open+0x78/0x59c [iuu_phoenix]

Fixes: 07c3b1a1

 ("USB: remove broken usb-serial num_endpoints
check")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Johan Hovold <johan@kernel.org>