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>