device: Update SYS_INIT_NAMED() to be C++ compatible

In #84394, `struct init_entry` was modified to remove an unneeded union.
The `SYS_INIT_NAMED()` macro was adjusted accordingly, but is no longer
C++ compatible due to the partial designated initializer.

Add an explicit value (NULL) for the other field (`dev`) in that struct.

Signed-off-by: Tristan Honscheid <honscheid@google.com>