spi: a3700: Clear DATA_OUT when performing a read

When performing a read using FIFO mode, the spi controller shifts out
the last 2 bytes that were written in a previous transfer on MOSI.

This undocumented behaviour can cause devices to misinterpret the
transfer, so we explicitly clear the WFIFO before each read.

This behaviour was noticed on EspressoBin.

Signed-off-by: Maxime Chevallier <maxime.chevallier@smile.fr>
Signed-off-by: Mark Brown <broonie@kernel.org>