mmc: rockchip_sdhci: Correct error checking

A pointer can not be negative. Use macro IS_ERR_OR_NULL() for checking.

Signed-off-by: Haolin Li <li.haolin@qq.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>