Fix opcode validation for OpOobCommand.command

The 'command' attribute of the OpOobCommand command is defined with a
default value of None, but its validation requires a member of
constants.OOB_COMMANDS, which doesn't accept None. This result in the
following error when submitting an opcode without the command:

  error type: wrong_input, error details:
  Parameter 'OP_OOB_COMMAND.command' fails validation

I suspect this was simply a mistake, since the commit that introduced
it (65e183af, “opcodes: Add opcode parameter definitions”) did lots of
bulk updates.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>