mmc: dw_mmc-rockchip: fix failing to mount partition with "discard"

Without MMC_CAP_ERASE support, we fail to mount partition
with "discard" option since mmc_queue_setup_discard is limited
for checking mmc_can_erase. Without doing mmc_queue_setup_discard,
blk_queue_discard fails to test QUEUE_FLAG_DISCARD flag, so we get
the following log from f2fs(actually similar to other file system):

mounting with "discard" option, but the device does not support discard

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>