mmc: dwmmc: Change designware MMC 'clksel' callback function to return status

Change 'clksel' callback function to allow the code to return a
status.

This patch is a preparation for enabling Arm-Trusted-Firmware (ATF)
in Intel SoC FPGA. This patch does not change functionality.

When using Arm-Trusted-Firmware (ATF) in Intel SoC FPGA, the MMC clock
related register is secure register which is required to be written
via SMC/PCSI call. It is possible that U-Boot fail to write the
register if there is unexpected error between U-Boot and ATF.
As a result, there maybe signal integrity on MMC connection due to
clock. So, the code should reports error to user when 'clksel' fail.

Signed-off-by: Siew Chin Lim <elly.siew.chin.lim@intel.com>
diff --git a/drivers/mmc/nexell_dw_mmc.c b/drivers/mmc/nexell_dw_mmc.c
index 753c89d..2723e48 100644
--- a/drivers/mmc/nexell_dw_mmc.c
+++ b/drivers/mmc/nexell_dw_mmc.c
@@ -51,7 +51,7 @@
 
 struct clk *clk_get(const char *id);
 
-static void nx_dw_mmc_clksel(struct dwmci_host *host)
+static int nx_dw_mmc_clksel(struct dwmci_host *host)
 {
 	/* host->priv is pointer to "struct udevice" */
 	struct nexell_dwmmc_priv *priv = dev_get_priv(host->priv);
@@ -65,6 +65,8 @@
 		      DWMCI_SET_DRV_CLK(DWMCI_SHIFT_0) | DWMCI_SET_DIV_RATIO(3);
 
 	dwmci_writel(host, DWMCI_CLKSEL, val);
+
+	return 0;
 }
 
 static void nx_dw_mmc_reset(int ch)