clk: sunxi: sun9i-mmc: check return value after calling platform_get_resource()

It will cause null-ptr-deref if platform_get_resource() returns NULL,
we need check the return value.

Fixes: 7a6fca87

 ("clk: sunxi: Add driver for A80 MMC config clocks/resets")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Link: https://lore.kernel.org/r/20220421134308.2885094-1-yangyingliang@huawei.com