ASoC: arizona: Fix type of clock rate pointer in arizona_set_sysclk

Both the sysclk and asyncclk members of arizona_priv are signed by we
refer to them through an unsigned pointer. This patch fixes this small
harmless error.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>