mmc: host: Fix spinbug in performance sysfs nodes

In CMDQ, the mmc_release_host runs in softirq context.
There's a potential race condition between show/set_perf
since it doesn't disable pre-emption. Change this to
disable preemption.

Change-Id: I20918a459e8b35ac666971b8ebf179f44aa9c40f
Signed-off-by: Asutosh Das <asutoshd@codeaurora.org>
Signed-off-by: Venkat Gopalakrishnan <venkatg@codeaurora.org>