usb gadget serial: use composite gadget framework

This switches the serial gadget over to using the new "function"
versions of the serial port interfacing code.  The remaining code
in the main source file is quite small...

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>