net: wifi: shell: Remove obsolete subcmd settings from commands

If a shell command does not have a sub command, then the 3rd parameter
to SHELL_SUBCMD_ADD() macro should be set to NULL so that the help
will only print information specific to that command and not all
wifi commands.

Fixes #82633

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>