staging: vchiq_2835_arm: Fix NULL ptr dereference in free_pagelist

This fixes a NULL pointer dereference on RPi 2 with multi_v7_defconfig.
The function page_address() could return NULL with enabled CONFIG_HIGHMEM.
So fix this by using kmap() instead.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Fixes: 71bad7f0

 ("staging: add bcm2708 vchiq driver")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>