spi: spi-cadence: Interleave write of TX and read of RX FIFO

spi: spi-cadence: Interleave write of TX and read of RX FIFO

When working in slave mode it seems the timing is exceedingly tight.
The TX FIFO can never empty, because the master is driving the clock so
zeros would be sent for those bytes where the FIFO is empty.

Return to interleaving the writing of the TX FIFO and the reading
of the RX FIFO to try to ensure the data is available when required.

Fixes: a84c11e16dc2 ("spi: spi-cadence: Avoid read of RX FIFO before its ready")
Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Link: https://lore.kernel.org/r/20230518093927.711358-1-ckeepax@opensource.cirrus.com
Signed-off-by: Mark Brown <broonie@kernel.org>