mmc: bcm2835: Don't overwrite max frequency unconditionally

mmc: bcm2835: Don't overwrite max frequency unconditionally

commit 118032be389009b07ecb5a03ffe219a89d421def upstream.

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

Fixes: 660fc733bd74 ("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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>