spi: core: Use DEVICE_ATTR_RW() for SPI slave control sysfs attribute

Convert the SPI slave control sysfs attribute from DEVICE_ATTR() to
DEVICE_ATTR_RW(), to reduce boilerplate.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20190731124738.14519-1-geert+renesas@glider.be


Signed-off-by: Mark Brown <broonie@kernel.org>