staging: rtlwifi: replace ---help--- with help in Kconfig

This patch fixes the checkpatch.pl warning:

WARNING: prefer 'help' over '---help---' for new help texts

Signed-off-by: Cezary Kierzyk <virdipravum.n008d3v@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>