mmc: do not pack random requests

Packed commands causes higher latency since the completion of each
request is sent to the upper layer upon completion of the complete
packed request.
The benefit from this feature is card dependent. Some of the card
vendors do not have any benefit from using packed commands for random
requests. In case there is no benefit in random requests packing,
it is better to disable the packing to prevent this high latency.
This patch also add the new stop packing reason to the write packing
statistics.

Change-Id: I9fad312ffefef8033feb9adcefa8e0d7b35c8308
Signed-off-by: Maya Erez <merez@codeaurora.org>