USB: serial gadget: remove needless data structure

This removes a needless data structure from the serial gadget code;
it's a small code shrink, and a larger data shrink.

Since "struct usb_request" already has a "struct list_head" reserved
for use by gadget drivers, the serial gadget code doesn't need to
allocate wrapper structs to hold that list ... it can (and should!)
just use the list_head provided for that exact use.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Cc: Al Borchers <alborchers@steinerpoint.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>