mmc: dw_mmc: exynos: fix the NULL pointer dereference error

"host->cur_slot" should be assigned to start the request.
So it can be the NULL pointer. This patch fixed this error.
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

drivers/mmc/host/dw_mmc-exynos.c

@@ -157,7 +157,7 @@ static void dw_mci_exynos_set_clksel_timing(struct dw_mci *host, u32 timing)
 	 * HOLD register should be bypassed in case there is no phase shift
 	 * applied on CMD/DATA that is sent to the card.
 	 */
-	if (!SDMMC_CLKSEL_GET_DRV_WD3(clksel))
+	if (!SDMMC_CLKSEL_GET_DRV_WD3(clksel) && host->cur_slot)
 		set_bit(DW_MMC_CARD_NO_USE_HOLD, &host->cur_slot->flags);
 }