spi: amd: Don't wait for a write-only transfer to finish

Return from a write-only transfer without waiting for
it to finish
But wait before a new transfer as the previous may
still happening and also wait before reading the data
from the FIFO

Signed-off-by: Lucas Tanure <tanureal@opensource.cirrus.com>
Reviewed-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Link: https://lore.kernel.org/r/20210910111529.12539-4-tanureal@opensource.cirrus.com
Signed-off-by: Mark Brown <broonie@kernel.org>