spi: spi-gpio: Don't set MOSI as an input if not 3WIRE mode

spi: spi-gpio: Don't set MOSI as an input if not 3WIRE mode

[ Upstream commit 3a6f994f848a69deb2bf3cd9d130dd0c09730e55 ]

The addition of 3WIRE support would affect MOSI direction even
when still in standard (4 wire) mode. This can lead to MOSI being
at an invalid logic level when a device driver sets an SPI
message with a NULL tx_buf.

spi.h states that if tx_buf is NULL then "zeros will be shifted
out ... " If MOSI is tristated then the data shifted out is subject
to pull resistors, keepers, or in the absence of those, noise.

This issue came to light when using spi-gpio connected to an
ADS7843 touchscreen controller. MOSI pulled high when clocking
MISO data in caused the SPI device to interpret this as a command
which would put the device in an unexpected and non-functional
state.

Fixes: 4b859db2c606 ("spi: spi-gpio: add SPI_3WIRE support")
Fixes: 5132b3d28371 ("spi: gpio: Support 3WIRE high-impedance turn-around")
Signed-off-by: Kris Bahnsen <kris@embeddedTS.com>
Link: https://lore.kernel.org/r/20221207230853.6174-1-kris@embeddedTS.com
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>