mmc: dw_mmc: Add locking around cmd11 timer

It is possible for the cmd11 interrupt to fire and delete the
cmd11_timer before the cmd11_timer was actually setup.  Let's fix this
race by adding a few spinlocks.  Note that the race wasn't seen in
practice without adding some printk statements, but it still seems
wise to fix.

Fixes: 5c935165 ("mmc: dw_mmc: Add a timeout for sending CMD11")
Signed-off-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>