spi: atmel: fix gpio chip-select in case of non-DT platform

The non-DT platform that uses this driver (actually the AVR32) was taking a bad
branch for determining if the IP would use gpio for CS.
Adding the presence of DT as a condition fixes this issue.

Fixes: 48203034 ("spi: atmel: add support for the internal chip-select of the spi controller")
Reported-by: Mans Rullgard <mans@mansr.com>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>
[nicolas.ferre@atmel.com: extract from ml discussion]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Tested-by: Mans Rullgard <mans@mansr.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org