mmc: sdhci: Get rid of redundant BUG_ONs

BUG is never the right thing for SDHCI to do. Get rid of BUG_ON in cases it
will oops anyway if the pointer is NULL, or if the condition is logically
impossible.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>