mmc: bcm2835: Drop pointer to mmc_host from bcm2835_host

The BCM2835 MMC host driver uses a pointer to get from the private
bcm2835_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: Lukas Wunner <lukas@wunner.de>
Cc: Frank Pavlic <f.pavlic@kunbus.de>
Cc: Alexander Graf <agraf@suse.de>
Reviewed-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>