spi: dw: Fix non-DMA transmit-only transfers

Ensure the transmit FIFO has emptied before ending the transfer by
dropping the TX threshold to 0 when the last byte has been pushed into
the FIFO. Include a similar fix for the non-IRQ paths.

See: https://github.com/raspberrypi/linux/issues/6285


Fixes: 6014649d ("spi: dw: Save bandwidth with the TMOD_TO feature")
Signed-off-by: Phil Elwell <phil@raspberrypi.com>