DW SPI: fix tx data loss on FIFO flush

In current implementation if some data still exists in Tx FIFO it
can be silently flushed, i.e. dropped on disabling of the controller,
which happens when writing 0 to DW_SPI_SSIENR (it happens in the
beginning of new transfer)

So add wait for current transmit operation to complete to be sure
that current transmit operation is finished before new one.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>