dwmmc: remove recursive FIFO threshold setup

Removed code works properly only once after power-up because on every
"first" invocation of "dwmci_init" existing value of "fifo_size" is used
for calculation of itself. More over other bits in the same register
(namely TX watermark and burst size) get corrupted (lost forever till
the next power-toggle).

I understand that usually (in production systems) U-Boot starts only
once and then passes control to Linux kernel etc. But when one does
U-Boot debugging it might be loaded in memory via JTAG multiple times.

Also I wasn't able to find any instance of "host->fifoth_val" usage so I
believe it's safe to just remove this code completely.

Another important thing - in the latest Linux kernel driver for DW MMC
comtroller I don't see any utilization (configuration/modificatoin) of
the register in question (DWMCI_FIFOTH). That's why I think we may want
to keep it untouched as well so we don't force a situation when kernel
driver gets DW MMC unexpectedly configured by U-Boot.

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