device.h: conditionally remove init braces for C++20 compatibility

Conditionally remove braces for designated initializers of anonymous
unions. This makes it compatible with C++20 while not breaking pre-C11
gcc.

This does for device.h what commit c15f029a ("init.h: restore
designated initializers in SYS_INIT_NAMED()") did for init.h

See https://docs.zephyrproject.org/latest/develop/languages/cpp/

 and
long discussion in #69411 for more obscure C/C++ compatibility details.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>