usb gadget: link fixes for serial gadget

Change how the serial gadget driver builds:  don't use
separate compilation, since it works poorly when key parts
are library code (with init sections etc).  Instead be as
close as we can to "gcc --combine ...".

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