usb : usb_kbd : Populating 'priv' member of USB keyboard device_t structure

This patch populates the 'priv' field of the USB keyboard device_t
structure. The 'priv' field is populated with the address of the
'struct usb_device' structure that represents the USB device.

The 'priv' field can then be used in the 'usb_event_poll' function to
determine the USB device that requires to be polled. An
example of its usage in 'usb_event_poll' function is as below.

	device_t *dev;
	struct usb_device *usb_kbd_dev;

	<snip>

	dev = device_get_by_name("usbkbd");
	usb_kbd_dev = (struct usb_device *)dev->priv;
	iface = &usb_kbd_dev->config.if_desc[0];

Signed-off-by: Thomas Abraham <t-abraham@ti.com>
Signed-off-by: Remy Bohmer <linux@bohmer.net>