mmc: card: modifying the trigger to start packing

The value of num_wr_reqs_to_start_packing which is the number
of write requests that triggers the write packing, should be
determined also by the ext_csd.max_packed_writes that is read
from the device and anyway, will not exceed
DEFAULT_NUM_REQS_TO_START_PACK.

Change-Id: If3494da8396bedd9c2a419974ce0c155ee591e41
Signed-off-by: Yaniv Gardi <ygardi@codeaurora.org>