EmbeddedPkg/DwEmmc: Adjust FIFO threshold

EmbeddedPkg/DwEmmc: Adjust FIFO threshold

Adjust FIFO threshold according to FIFO depth. Skip
the adjustment if we do not have FIFO depth info.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jun Nie <jun.nie@linaro.org>
Acked-by: Haojian Zhuang <haojian.zhuang@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>