log: fix default level

The initialization of the default log level (from configuration) was
removed by mistake in a previous commit. The global log level was
wrongly set to debug when no --log-level argument was passed. Restore
this initialization.

Before:
  $ ./build/app/test
  RTE>>dump_log_types
  global log level is debug
  ...

After:
  $ ./build/app/test
  RTE>>dump_log_types
  global log level is info
  ...

Fixes: 845afe51e428 ("eal: change specific log levels at startup")

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