log: fix dump of registered logs when disabled

log: fix dump of registered logs when disabled

When we pass --log-level=0, it disables the logs. This level is
not displayed properly by the function that dumps the registered log
types (it shows "unknown"). Show "disabled" instead.

Before:

  ./build/app/test --log-level=0
  RTE>>dump_log_types
  global log level is unknown
  ...

After:

  ./build/app/test --log-level=0
  RTE>>dump_log_types
  global log level is disabled
  ...

Fixes: 432050bfd05b ("eal: dump registered log types")

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>