mmc: core: Restrict number of request in driver queue

When voltage corner is in LSVS, on low load scenario and
there is sudden burst of requests, all device queue
slots are filled and it is needed to wait till all
requests are completed to scale up frequency. This
is leading to delay in scaling and impacting performance.
Fix this issue by only allowing one request in request queue
when device is running with lower speed mode.

Ref: 01deb37c7 on msm-4.9 kernel.

Change-Id: Ie73d29095d9fdd48f1f6aa1c8fa82921022137b9
Signed-off-by: Ram Prakash Gupta <rampraka@codeaurora.org>