mmc: dw_mmc: fix data starvation by host timeout under FIFO mode

mmc: dw_mmc: fix data starvation by host timeout under FIFO mode

This patch fixes data starvation by host timeout(HTO) error interrupt
which occurred under FIFO mode transfer on rk3036 board.

The former implement, the actual bytes were transmitted may be less than
should be. The size will still subtract value of len in case of there is
no receive/transmit FIFO data request interrupt.

Signed-off-by: Ziyuan Xu <xzy.xu@rock-chips.com>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>