spi: davinci: fix a NULL pointer dereference

On non-OF systems spi->controlled_data may be NULL. This causes a NULL
pointer derefence on dm365-evm.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org