usb: gadget: Quieten gadget config message

On a system that often re-configures a USB gadget device the kernel log
is filled with:

  configfs-gadget gadget: high-speed config #1: c

Reduce the verbosity of this print to debug.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>