spi/dw_spi: remove the un-necessary flush()

The flush() is used to drain all the left data in rx fifo, currently
is is always called together with disabling hw. But from spec, disabling
hw will also reset all the fifo, so flush() is not needed.

Signed-off-by: Alek Du <alek.du@intel.com>
Signed-off-by: Feng Tang <feng.tang@intel.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>