usb: gadget: inode: switch over to memdup_user()

This patch fixes the following Coccinelle warning:

drivers/usb/gadget/inode.c:442:8-15: WARNING \
	opportunity for memdup_user

Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>