mmc: dw_mmc: Fix out-of-bounds access for slot's caps

Add num_caps field for dw_mci_drv_data to validate the controller
id from DT alias and non-DT ways.

Reported-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Fixes: 800d78bf ("mmc: dw_mmc: add support for implementation specific callbacks")
Cc: <stable@vger.kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>