ARM: 8697/1: dma-mapping: Do not pass data to gen_pool_set_algo()

gen_pool_first_fit_order_align() does not make use of additional data,
so pass plain NULL there.

Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>