mmc: card: Fix packing control enabling algorithm

When hitting a stop potential packing event, we should zero
num_of_potential_packed_wr_reqs, so that the packing won't be
enabled too early.

Change-Id: I91d36765c4a50be0e2805cd07c58962fb87153c6
Signed-off-by: Maya Erez <merez@codeaurora.org>
Signed-off-by: Tatyana Brokhman <tlinder@codeaurora.org>