mmc: core: guard dereference of optional parameter

Various functions take as parameter an optional pointer. Pointer
should be guarded with non-NULL check before dereferencing.

Add non-NULL check before dereference of pointer.

Signed-off-by: Tobin C. Harding <me@tobin.cc>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>