spi: Make GPIO CSs honour the SPI_NO_CS flag

The SPI configuration state includes an SPI_NO_CS flag that disables
all CS line manipulation, for applications that want to manage their
own chip selects. However, this flag is ignored by the GPIO CS code
in the SPI framework.

Correct this omission with a trivial patch.

See: https://github.com/raspberrypi/linux/issues/2169



Signed-off-by: Phil Elwell <phil@raspberrypi.org>