spi: spi-oc-tiny: Use of_match_ptr() macro

This eliminates having an #ifdef returning NULL for the case
when OF is disabled.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Thomas Chou <thomas@wytron.com.tw>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>