mmc: core: capture performance numbers only when asked

Currently performance numbers are captured for each SDCC
transfers unconditionally which may add the overhead and
could reduce the SDCC read/write throughput numbers.

This change adds additional control for enabling/disabling the
capturing of performance numbers at runtime. We already have sysfs
entry named "perf" for msm sdcc devices. Currently setting this
entry to 0 clears the performance statistics. But now we are
changing the definition of this entry as mentioned below:

Disable performance capturing and clear the performance statistics:
	"echo 0 > /sys/devices/platform/msm_sdcc.<n>/perf"

Enable performance capturing:
	"echo 1 > /sys/devices/platform/msm_sdcc.<n>/perf"

CRs-fixed: 345170
Change-Id: I3ab9288fd87cc8a8ada6c0c3d066cac4f68d79b7
Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>