usertools: fix syntax warning in python 3.8

usertools: fix syntax warning in python 3.8

Silent the following warning when running script with python 3.8:

> /usr/bin/dpdk-pmdinfo:542: SyntaxWarning: "is" with a literal.
> Did you mean "=="?
>   if (autoload_path is None or autoload_path is ""):

As autoload_path can only be None or a string, directly check its bool
value.

Fixes: c67c9a5c646a ("tools: query binaries for HW and other support information")
Cc: stable@dpdk.org
Signed-off-by: Thomas Faivre <thomas.faivre@6wind.com>