usertools: fix pmdinfo parsing

usertools: fix pmdinfo parsing

In the display_pmd_info_strings function, the script parses the section
until to find a byte between 32 and 127, and get all data
until a byte equals to 0.
After, it searches "PMD_INFO_STRING" in the data and passes the whole
string in the parse_pmd_info_string function, which split the string
with "=" and convert it in python dict with json.loads().

But the string may contain a "=" before "PMD_INFO_STRING",
so it is not correctly split and will lead to an error
(json.decoder.JSONDecodeError).

Example of a string encountered that leads to an error:

"Ag%=C£°ÐÊ+Ë®{0´wË-£0òjB·;¾¬úPMD_INFO_STRING= {"name" :
"net_octeontx", "params" : "nr_port=<int> ", "pci_ids" : []}"

Fixes: c67c9a5c646a ("tools: query binaries for HW and other support information")
Cc: stable@dpdk.org
Signed-off-by: Julien Massonneau <julien.massonneau@6wind.com>