spi: loopback-test: implement testing with no CS

To run spi-loopback-tests on HW without modifications, we need to
disable Chip Select. This should avoid surprising side effects for
SPI devices by testing patterns.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Mark Brown <broonie@kernel.org>