usertools/pmdinfo: fix plugin auto scan

usertools/pmdinfo: fix plugin auto scan

Migration to argparse was incomplete.

$ dpdk-pmdinfo.py -p $(which dpdk-testpmd)
Traceback (most recent call last):
  File "/usr/bin/dpdk-pmdinfo.py", line 626, in <module>
    main()
  File "/usr/bin/dpdk-pmdinfo.py", line 596, in main
    exit(scan_for_autoload_pmds(args[0]))
TypeError: 'Namespace' object does not support indexing

Fixes: 81255f27c65c ("usertools: replace optparse with argparse")
Cc: stable@dpdk.org
Signed-off-by: David Marchand <david.marchand@redhat.com>
Reviewed-by: Robin Jarry <robin.jarry@6wind.com>