mmc: sdhci-pxav3: Remove checks for optional core clock in error/remove path

Commit 63589e92

 ("clk: Ignore error and NULL pointers passed to
clk_{unprepare, disable}()") allows NULL or error pointer to be passed
unconditionally.

This patch is to simplify probe error and remove code paths.

However, we reserve the core clock checks in runtime suspend/resume code
because we want a little smaller latency.

Signed-off-by: Jisheng Zhang <jszhang@marvell.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>