device: add braces around dev/dev_rw initializer

The init_entry struct got modified to add a union with a non const dev
pointer in afc59112. Some old compiler (such as GCC 4) seems to
require a pair of brackets to correctly initialize the field in the
union. Add those brackets to the initializers in device.h and init.h to
maintain compatibility.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>