spi: s3c64xx: clear loopback bit after loopback test

[ Upstream commit 9ec3c551

 ]

When SPI loopback transfer is performed, S3C64XX_SPI_MODE_SELF_LOOPBACK
bit still remained. It works as loopback even if the next transfer is
not spi loopback mode.
If not SPI_LOOP, needs to clear S3C64XX_SPI_MODE_SELF_LOOPBACK bit.

Signed-off-by: Jaewon Kim <jaewon02.kim@samsung.com>
Fixes: ffb7bcd3

 ("spi: s3c64xx: support loopback mode")
Reviewed-by: Chanho Park <chanho61.park@samsung.com>
Link: https://lore.kernel.org/r/20230711082020.138165-1-jaewon02.kim@samsung.com
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>