arm: rpi: fallback to max clock rate for MMC clock

arm: rpi: fallback to max clock rate for MMC clock

In rpi-firmware 25e2b597ebfb2495eab4816a276758dcc6ea21f1,
the GET_CLOCK_RATE mailbox property was changed to return the last
value set by SET_CLOCK_RATE.

https://github.com/raspberrypi/firmware/issues/1619#issuecomment-917025502

Due to this change in firmware behavior, bcm2835_get_mmc_clock now
returns a clock rate of zero since we do not issue SET_CLOCK_RATE.
This results in degraded MMC performance.

SET_CLOCK_RATE fixes the clock to a specific value and disables scaling
so is not an ideal solution.

Instead, fallback to GET_MAX_CLOCK_RATE in bcm2835_get_mmc_clock if
GET_CLOCK_RATE returns zero.

Signed-off-by: Vincent Fazio <vfazio@xes-inc.com>
Signed-off-by: Peter Robinson <pbrobinson@gmail.com>