mmc: sdhci: Clear SDHCI_CLOCK_CONTROL before configuring the new value

This patch completely clears the SDHCI_CLOCK_CONTROL register before the
new value is configured instead of just clearing the 2 bits
SDHCI_CLOCK_CARD_EN and SDHCI_CLOCK_INT_EN. Without this change, some
clock configurations will lead to the "Internal clock never stabilised."
error message on the Xenon SDHCI controller used on the Marvell Armada
3700 and 7k/8k ARM64 SoCs.

The Linux SDHCI core driver also writes 0 to this register before
the new value is configured. So this patch simplifies the driver a bit
and brings the U-Boot driver more in-line with the Linux one.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Jaehoon Chung <jh80.chung@samsung.com>
Cc: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Cc: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>

drivers/mmc/sdhci.c

@@ -295,7 +295,7 @@ static int sdhci_send_command(struct mmc *mmc, struct mmc_cmd *cmd,
 static int sdhci_set_clock(struct mmc *mmc, unsigned int clock)
 {
 	struct sdhci_host *host = mmc->priv;
-	unsigned int div, clk = 0, timeout, reg;
+	unsigned int div, clk = 0, timeout;
 
 	/* Wait max 20 ms */
 	timeout = 200;
@@ -311,9 +311,7 @@ static int sdhci_set_clock(struct mmc *mmc, unsigned int clock)
 		udelay(100);
 	}
 
-	reg = sdhci_readw(host, SDHCI_CLOCK_CONTROL);
-	reg &= ~(SDHCI_CLOCK_CARD_EN | SDHCI_CLOCK_INT_EN);
-	sdhci_writew(host, reg, SDHCI_CLOCK_CONTROL);
+	sdhci_writew(host, 0, SDHCI_CLOCK_CONTROL);
 
 	if (clock == 0)
 		return 0;