MMC: DWMMC: Fix FIFO_DEPTH calculation

Current DWMMC driver used to give FIFO underrun/overrun error every 3rd time
for mmc rescan command.
In current code FIFO_DEPTH is getting calculated after reading the default FIFOTH
register and extracting the RX_WMARK bits from it i.e (RX_WMARK = FIFO_DEPTH/2 -1).
Instead of storing the correct value, we were recalculating the FIFO_DEPT each
time which is not correct.

Based on "[PATCH V9 3/9] DWMMC: Initialise dwmci and resolve EMMC read write issues"
http://permalink.gmane.org/gmane.comp.boot-loaders.u-boot/160247

Signed-off-by: Hatim Ali <hatim.rv@samsung.com>
Signed-off-by: Rajeshwari Shinde <rajeshwari.s@samsung.com>
Acked-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Acked-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>