aubinator: Reuse decode_structure code for handling commands

aubinator: Reuse decode_structure code for handling commands

The code for decoding structures and commands was almost identical.
The only differences are: we print dword headers for commands, and
we skip the first one (with the command opcode and lengths).

So, generalize decode_structure to add a starting DWord, and a flag
for printing the DWord headers, and reuse it.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>