mmc: sdhci: Tuning should not change max_blk_count

Re-tuning requires that the maximum data length
is limited to 4MiB. The code currently changes
max_blk_count in an attempt to achieve that.
This is wrong because max_blk_count is a different
limit, but it is also un-necessary because
max_req_size is 512KiB anyway. Consequently, the
changes to max_blk_count are removed and the
comment for max_req_size adjusted accordingly.
The comment is also tweaked to show that the 512KiB
limit is a SDMA limit not an ADMA limit.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Reviewed-by: Aaron Lu <aaron.lu@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>