staging: most: usb: Change '--help--' to 'help' in Kconfig

Change '--help--' in Kconfig file to silence checkpatch.pl warning:
WARNING: prefer 'help' over '---help---' for new help texts

Signed-off-by: Wentao Cai <etsai042@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>