mmc: dw_mmc: fix num_slots setting

This patch make num_slots to 1 if pdata->num_slot is not
defined. Meanwhile, we need to make sure num_slots should
not larger that the supported slots

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>