mmc: bcm2835: Don't overwrite max frequency unconditionally

The optional DT parameter max-frequency could init the max bus frequency.
So take care of this, before setting the max bus frequency.

Fixes: 660fc733

 ("mmc: bcm2835: Add new driver for the sdhost controller.")
Signed-off-by: Phil Elwell <phil@raspberrypi.org>
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Cc: <stable@vger.kernel.org> # 4.12+
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>