mmc: sdhci-msm: Reduce the max descriptors

8K is the max descriptors supported by the hardware per transfer.
On normal use cases we never use that many descriptors even if
the memory is extremely fragmented. The memory allocated for these
descriptors are never freed hence wasting memory otherwise
available for the system. Reduce the max descriptors to 512 to save
memory (128K with 8K descriptors vs 8K with 512 descriptors) and
still support maximum performance.

CRs-fixed: 684185
Change-Id: Ib57ce03834d741c0fda4195a58d6b287ee9fb0a0
Signed-off-by: Venkat Gopalakrishnan <venkatg@codeaurora.org>