usb: Fix warnings in chaoskey driver

>    drivers/usb/misc/chaoskey.c: In function 'chaoskey_read':
> >> drivers/usb/misc/chaoskey.c:412:3: error: implicit declaration of function 'copy_to_user'
> >> [-Werror=implicit-function-declaration]
>       remain = copy_to_user(buffer, dev->buf + dev->used, this_time);

I was unable to reproduce this locally, but added an explicit

	#include <linux/uaccess.h>

which should ensure the definition on all architectures.

> sparse warnings: (new ones prefixed by >>)
>
> >> drivers/usb/misc/chaoskey.c:117:30: sparse: incorrect type in assignment (different base types)
>    drivers/usb/misc/chaoskey.c:117:30:    expected int [signed] size
>    drivers/usb/misc/chaoskey.c:117:30:    got restricted __le16 [usertype] wMaxPacketSize

Switched the code to using the USB descriptor accessor functions.

Signed-off-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>