mmc: core: use non-reentrant workqueue for clock gating

The default multithread workqueue can cause the same work to be executed
concurrently on a different CPUs. This isn't really suitable for clock
gating as it might already gated the clock and gating it twice results both
host->clk_old and host->ios.clock to be set to 0.

To prevent this from happening we use system_nrt_wq instead.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Tested-by: Chris Ball <cjb@laptop.org>
Cc: <stable@kernel.org>
Signed-off-by: Chris Ball <cjb@laptop.org>