USB: serial: Fix module name typo for qcaux Kconfig entry.

The module is called qcaux and not moto_modem. Also use help instead of
---help-- to be in sync with the other Kconfig entries.

Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>