USB: serial: move the "simple" drivers into usb-serial-simple.c

Instead of having to create a new driver for a "simple" usb to serial
device, mush them all into one file, with a macro, so as to make it easy
to add new ones.

Cc: "René Bürgel" <rene.buergel@sohard.de>
Acked-by: Wei Shuai <cpuwolf@gmail.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Acked-by: Frans Klaver <frans.klaver@xsens.com>
Cc: "Wesley W. Terpstra" <w.terpstra@gsi.de>
Cc: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>