mmc: alcor: Drop pointer to mmc_host from alcor_sdmmc_host

The driver for Alcor Micro AU6601 and AU6621 controllers uses a pointer to
get from the private alcor_sdmmc_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>