USB: serial: spcp8x5: 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 spcp8x5_open+0x30/0xd0 [spcp8x5]

Fixes: 619a6f1d ("USB: add usb-serial spcp8x5 driver")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Johan Hovold <johan@kernel.org>