mmc/dwmmc: modify FIFO threshold only if value explicitly set

If platform provides "host->fifoth_val" it will be used for
initialization of DWMCI_FIFOTH register. Otherwise default value will be
used.

This implementation allows:
 * escape unclear and recursive calculations that are currently in use
 * use whatever custom value for DWMCI_FIFOTH initialization if any
particular SoC requires it

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>

Cc: Mischa Jonker <mjonker@synopsys.com>
Cc: Alim Akhtar <alim.akhtar@samsung.com>
Cc: Rajeshwari Shinde <rajeshwari.s@samsung.com>
Cc: Jaehoon Chung <jh80.chung@samsung.com>
Cc: Amar <amarendra.xt@samsung.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Minkyu Kang <mk7.kang@samsung.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Pantelis Antoniou <panto@antoniou-consulting.com>
Cc: Andy Fleming <afleming@freescale.com>