fiptool: Invoke command specific usage function

Instead of always calling the top level usage function when an
error is detected, call the command-specific usage function.

For example running `fiptool create` will produce the same output
as `fiptool help create`.  This is more convenient for the user
when they make a mistake.

Change-Id: I60178ab89d47adf93cdfe6d8b5d5f778a5ea3bca