spi: Change FIFO flush operation and spi channel off

Setting SW_RST does TX/RX FIFO flush.
After FIFO flush, SW_RST should be cleared.
The above setting and clearing SW_RST operation should be done after spi channel off.

Signed-off-by: Kyoungil Kim <ki0351.kim@samsung.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>