spi: rb4xx: Fix set_cs logic.

As it turns out, the set_cs() enable parameter refers to the logic level
on the CS pin, not the state of chip selection.

This broke functionality of the LEDs behind the CPLD, or at least delayed
the commands until another one came in to toggle CS.

Signed-off-by: Bert Vermeulen <bert@biot.com>
Signed-off-by: Mark Brown <broonie@kernel.org>