spi: Force CS_HIGH if GPIO descriptors are used

Commit f3186dd8 ("spi: Optionally use GPIO descriptors for CS GPIOs")
amended of_spi_parse_dt() to always set SPI_CS_HIGH for SPI slaves whose
Chip Select is defined by a "cs-gpios" devicetree property.

This change breaks drivers whose probe functions set the mode field of
the spi_device because in doing so they clear the SPI_CS_HIGH flag.

Fix by setting SPI_CS_HIGH in spi_setup (under the same conditions as
in of_spi_parse_dt()).

See also: 83b2a8fe ("spi: spidev: Fix CS polarity if GPIO descriptors are used")

Fixes: f3186dd8

 ("spi: Optionally use GPIO descriptors for CS GPIOs")
Signed-off-by: Phil Elwell <phil@raspberrypi.com>