mmc: android-goldfish: Drop pointer to mmc_host from goldfish_mmc_host

The driver for android-goldfish uses a pointer to get from the private
goldfish_mmc_host structure to the generic mmc_host structure.
However the latter is always immediately preceding the former in
memory, so compute its address with a subtraction (which is cheaper than a
dereference) and drop the superfluous pointer.

No functional change intended.

Signed-off-by: Kamlesh Gurudasani <kamlesh.gurudasani@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>