mmc: rtsx_pci: Fix long reads when clock is prescaled

For unexplained reasons, the prescaler register for this device needs to
be cleared (set to 1) while performing a data read or else the command
will hang. This does not appear to affect the real clock rate sent out
on the bus, so I assume it's purely to work around a hardware bug.

During normal operation, the prescaler is already set to 1, so nothing
needs to be done. However, in "initial mode" (which is used for sub-MHz
clock speeds, like the core sets while enumerating cards), it's set to
128 and so we need to reset it during data reads. We currently fail to
do this for long reads.

This has no functional affect on the driver's operation currently
written, as the MMC core always sets a clock above 1MHz before
attempting any long reads. However, the core could conceivably set any
clock speed at any time and the driver should still work, so I think
this fix is worthwhile.

I personally encountered this issue while performing data recovery on an
external chip. My connections had poor signal integrity, so I modified
the core code to reduce the clock speed. Without this change, I saw the
card enumerate but was unable to actually read any data.

Writes don't seem to work in the situation described above even with
this change (and even if the workaround is extended to encompass data
write commands). I was not able to find a way to get them working.

Signed-off-by: Thomas Hebb <tommyhebb@gmail.com>
Link: https://lore.kernel.org/r/2fef280d8409ab0100c26c6ac7050227defd098d.1627818365.git.tommyhebb@gmail.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>