mmc: dw_mmc: fix the wrong condition check of getting num-slots from DT

Change to print the information about when the deprecated "num-slots" DT
binding is being used, as to avoid confusion when browsing the log:

dwmmc_rockchip fe320000.dwmmc: 'num-slots' was deprecated.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Fixes: d30a8f7b

 ("mmc: dw_mmc: deprecated the "num-slots" property")
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>