mmc: core: Fix debounce time to use microseconds

The debounce value in device tree is in milliseconds but needs to be in
microseconds for mmc_gpiod_request_cd().

Fixes: bfd694d5 ("mmc: core: Add tunable delay before detecting card
after card is inserted")
Cc: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Cc: stable@vger.kernel.org # v4.18+
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>