libubus: Make UBUS_* macros work cleanly in C++

libubus: Make UBUS_* macros work cleanly in C++

C++ is picky about initializer order, and (depending on flags) missing fields.
This fix makes UBUS_METHOD_* and UBUS_OBJECT_TYPE initialize all fields of the
respective structs in the correct order.

Also replace BIT(x) with an explicit expression since BIT() may not be defined.

Signed-off-by: Karsten Sperling <ksperling@apple.com>