spi: efm32: Clean up non-DT paths

This is a DT-only driver, so remove all non-DT paths.
of_gpio_named_count() may fail, thus also add checking it's return value.
efm32_spi_probe_dt() never fails, make it return void.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Mark Brown <broonie@linaro.org>