mmc: dw_mmc: Use core to handle absent write protect line

Use the new MMC_CAP2_NO_WRITE_PROTECT to let the core handle the case where
no write protect line is present instead of having custom driver code to
handle it.

dw_mci_of_get_slot_quirks() is slightly refactored to directly modify the
mmc_host capabilities instead of returning a quirk mask.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>