thunderbolt: Use NULL instead of 0 in switch.c

The function returns a pointer. Hence return NULL instead of 0.

Signed-off-by: Sachin Kamat <sachin.kamat@samsung.com>
Acked-by: Andreas Noever <andreas.noever@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>