USB: gadget: Fix mistakes in UDC core kerneldoc

This patch fixes some minor mistakes in the UDC core's kerneldoc.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Link: https://lore.kernel.org/r/YmSpKpnWR8WWEk/p@rowland.harvard.edu
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>