spi: sc18is602: Remove sc18is602_setup() because it is done by spi core

The checking for spi->mode is done in the implementation of spi_setup().
Calling sc18is602_check_transfer(spi, NULL, 0) is pointless because
the code is equivent to checking if spi->max_speed_hz is 0.
Note, sc18is602_check_transfer actually allows spi->max_speed_hz is 0
if t->speed_hz is set. So return error in sc18is602_setup() when
spi->max_speed_hz is 0 does not make sense.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Mark Brown <broonie@linaro.org>