spi: gpio: Don't request CS GPIO in DT use-case

DT use-case already relies on SPI core to control CS (requested by
of_spi_register_master() and controlled spi_set_cs()), so there's no
need to try to request those GPIO in spi-gpio code. Change the code
such that spi-gpio's CS related code is only used if device is probed
via pdata.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Chris Healy <cphealy@gmail.com>
Cc: linux-spi@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>