xtensa: ISS: fix call to split_if_spec

split_if_spec expects a NULL-pointer as an end marker for the argument
list, but tuntap_probe never supplied that terminating NULL. As a result
incorrectly formatted interface specification string may cause a crash
because of the random memory access. Fix that by adding NULL terminator
to the split_if_spec argument list.

Cc: stable@vger.kernel.org
Fixes: 7282bee7 ("[PATCH] xtensa: Architecture support for Tensilica Xtensa Part 8")
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>