usb: gadget: composite: use kasprintf() instead of open coding it

Let's not reimplement generic kernel helpers,
instead call kasprintf().

[ felipe.balbi@linux.intel.com: better commit log ]

Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>