mmc: dw_mmc: Factor out dw_mci_init_slot_caps

Factor out dw_mci_init_slot_caps to consolidate parsing
all differents types of capabilities from host contrllers.
No functional change intended.

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>