mmc: dw_mmc: empty FIFO after data transfer over interrupt in pio mode

In dwc manual, the below contents are described:

"During end of packet, interrupt is not generated if threshold
programming is larger than any remaining data. It is responsibility
of host to read remaining bytes on seeing Data Transfer Done
interrupt"

We also have seen the data cannot be read fully when
"sg_miter->length" is less than FIFO size.

Signed-off-by: Kyoungil Kim <ki0351.kim@samsung.com>
Signed-off-by: Seungwon Jeon <tgih.jun@samsung.com>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Chris Ball <cjb@laptop.org>