trace: remove unneeded checks in internal API

trace: remove unneeded checks in internal API

The trace framework can be configured via 4 EAL options:
- --trace which calls eal_trace_args_save,
- --trace-dir which calls eal_trace_dir_args_save,
- --trace-bufsz which calls eal_trace_bufsz_args_save,
- --trace-mode which calls eal_trace_mode_args_save.

Those 4 internal callbacks are getting passed a non NULL value:
optarg won't be NULL since those options are declared with
required_argument (man getopt_long).

eal_trace_bufsz_args_save() already trusted passed value, align the other
3 internal callbacks.

Coverity issue: 357768
Fixes: 8c8066ea6a7b ("trace: add trace mode configuration parameter")
Signed-off-by: David Marchand <david.marchand@redhat.com>
Acked-by: Sunil Kumar Kori <skori@marvell.com>