mmc: card: update long_sequential_write_test queue depth

Since some systems have higher memory pressure, we limit the amount of
simultaneous test requests. The former limitation was 118, due to the
request queue size, but now we further restrict this number to half.

Change-Id: I893b9ad1b133c922f90497b0ec1f416514529fb8
CRs-fixed: 510814
Signed-off-by: Lee Susman <lsusman@codeaurora.org>